I've never had a problem like this so I can only guess, but as far as I know there's only desc76 and desc77 in the magic data file that determines when the CC cars stop to change tyres in the rain.
Default values seem to range from 9373 to 15453 for desc76 and 13000 to 24000 for desc77
Higher values mean change sooner, so I would use cmagic4 to extract a fresh copy of the MD from the track you're using and took a look at those values - and if they aren't within those ranges try changing them to something like:
15000 ; ????? - helps decide if a tyre set needs changing? (dw)
24000 ; cc crossover point (higher, sooner)
If that didn't fix it, I would extract a fresh copy of the MD from another version of the same track and use that.
Beyond that, I've no idea...