B-spec drivers

put them in a slower car or tune the one they are in so they don't spin

Calm his pace down after he gets the lead then modify as needed during the race.

I moved him from the mazda to the '62 buick special, at 850HP and he seems to be doing fine now. I don't understand, but at least he is winning now.

definitely "pace down" when he's out in front of the pack. once your driver is at a higher level, you shouldn't have to manage him quite so much. it also has to do with the personality of the driver you initially chose - if he's hot (red) he'll drive aggressively to take the lead at the expense of control. if he's cold (blue) he's drive more conservatively.

i've also found this guide helpful:

https://www.gtplanet.net/forum/showthread.php?t=139743

my b-spec driver is at level 15 and it is a great way to earn easy credits and prize cars.