I don't know if anyone else has experienced this issue, but just in case they have, I am posting my latest find which appears to have fixed it.
I have several installs of SLRR on my PC, but my preference is MWM221LE2.. and this is the version I am discussing here.
I really like the little Fiat 600, but I have had a problem with it.. as the standard engine is next to useless for racing, and cannot be tuned or modified
apart from max RPM adjustments... I dump the engine and install anything else (Hayabusa is one favourite of mine).
The problem I have had (most of the time) is that this causes the car to bounce around when I take it to Valo night races
(or sometimes on the test track).
I read somewhere that this can be rectified by changing springs, or engine position, or playing around with weights.. but none of this fixed it for me.
Now, in this version of the game I never use the car jack as it crashes the game for me, but I have just found out that
with the Fiat the jack MUST be used when removing or replacing parts (this is one of the few cars that doesn't crash
the game when using the jack).
Eureka!! the car no longer bounces about at Valo, or on the test track.. no matter what engine or other mods I fit to it.
So in short... if you are having the same problem I had with this car, try using the jack before doing any changes to
the Fiat and then test it at the test track, or try it at Valo night races. It worked for me, and I hope it will work
for others.

# Bigg Boss93 : Where did you download the 600 from?

Hmm.. I honestly don't remember which site I got it from, BB... but it was either this one, or the other popular one GOM

That error is caused from too low physics(since 99.9% of the modders never cared about editing them), so they collide with other solid meshes(such as ground).
You can fix it by stripping the car(leaving only chassis, running gears, engine, driver seat, steering wheel and battery) and trying if it hits somewhere/bounces.

I'll give that a try and report back (I described the problem as 'bouncing', but it's more like driving with 'oval' or off-centre rear wheels really.. back of the car lifts and drops radically).

If it does then go to the car's "main.cfg" and edit the coordinates of its physics to be higher(we have a tutorial on coordinates on our tutorial section).

I've never tried messing with mods, but I'll give it a go as I'd rather fix the problem properly.. and gain some free education to boot!

Many thanks for the answer and guidance, btw.. I might be one of the oldest members on here, but a huge fan of the game

# Bigg Boss93 :...let me know how it goes

Well BB, I think I have found the cause of the little Fiat's problem... but fixing it is beyond me, apart from using the jack when I work on the car.
Over the past couple of months I've been busy with work, but now I have some R&R time, so I built up a new PC dedicated to SLRR for testing and enjoying the game, lol..
I loaded a fresh copy of SLRR 2.2.1 MWM LE2 (my preference out of all versions), then started loading the mods I like... Dealer mod, Adjustable fuel types, engine position mod, etc. Then came the cars, engines, wheels & tyres, suspension, etc. I only loaded mods that I know are stable and work well together, as I'm trying to build up a Compilation that doesn't crash when doing night time drag races at Valo. So far the game is around 2 Gigabytes in size and runs reliably every time. During my download sessions, I discovered another little Fiat which I hadn't used before.. the 126p, which comes in standard, Sporting and Racing versions... a fantastic little car, super light and with better handling than the FS600. During testing with this car, I found it suffered exactly the same way as the FS600... when not using the ramp to work on the car, it would jump around all over the place with each part I fitted (this was the first clue).
Even when removing seats or windows from the car, one of the wheels will start bouncing up and down, falling into the pit beneath the jack, then jumping back out again, repeatedly. Quite often, the car would drop into the pit and rest on it's side (with the wheel still bouncing around). I am now convinced that this is the main problem with both thie FS600 and the 126p Fiats... they are so narrow that they do not stay securely on the ground above the pit when being worked on (unless using the jack of course) and this causes damage to the wheels. To check this, I deliberately worked on each of these models without using the jack so that when I took them to the 'Test track' they both gave the 'Bouncing' problems I described earlier in this thread... then I took them back to the workshop, used the jack to lift them, and fitted new wheels and tyres. It worked! taking them back to the test track with new wheels and tyres fitted, the bouncing had stopped. So, I am sure that the wheels and tyres are being damaged when not using the jack to work on the car.
I will continue to evaluate this theory with any other small cars I can use, and further develop my compilation.. then I will upload it when completed so that any one looking for a 'Stable' version of the game can download and try it out.

But for now.. it's Play time!

Later..