XRAY - Model racing cars

If your rear diff is too tight this can be your problem.

If you use a diff in the front, this can be too loose, the result would be similar.

But there are other possibilities. For example ride height (rear too low), droop...

I had a similar issue.
I use the yellow springs (soft) and removed the antiroll bar on the rear.
Also higher pinion gear to get a bit less agressive is an option.

Now my car is running fine for that type of carpettrack

If the car is set up ok, then almost always it is the diff to blame for such thing. When diff doesn't work good you will lose grip at rear. But check that everything is straigt, arms are fine etc.

With onroad balldiffs it usually works best if it is as loose as possible without slipping. I don't know if there's a situation when tighter diff works better? I'm running rubber with front spool.