I see this all the time, you are definitely not alone. I have a guy I race with a lot, he does great when he has the lead, but if I get in front, hes sliding off every corner.
Just remember the track and remember your car's limits. Keep your car on the track, if the guy ahead of you is faster, his car may just be faster and there isn't much you can do about that except bide your time. Sometimes they will make a mistake and slide off themselves, and if you are there and in control, you can take their spot :-)
Patience is a big part of racing.