Thing is, I've noticed it happens more and more as I've got better at the game.
I don't think it's simply that the frame-rate is dropping off.
It only ever seems to happen to me when I've taken a strong lead or when I've made up a lot of places very quickly.
I've also noticed that when the frame-rate drops off the other cars suddenly seem to catch me up (even on the straights) and my lap-times drop off by as much as 20 seconds a lap.
I'm starting to think it's actually something to do with a "fiddle-factor" which is designed to allow the AI cars to catch up with the player.
It seems as though when you're simply TOO far in front for the rubber-band system to work it slows the entire game down for the player in order for the AI's to catch up again.
For example, I was playing last night and I LOVE the Montreal track. I was driving the Virgin and started in 16th place.
I was able to cut through the field, take the lead and pull out a 20 second gap.
Suddenly, at the hairpin, the frame-rate dropped, I continued along the final straight, through the last couple of corners and my lap-time said I was TWENTY SECONDS off the pace.
By the time I reach the first turns of the next lap all the following cars had caught up and I found myself in about 10th place.
The frame-rate remained terrible for about half a lap and then reverted to normal.
I did it all over again, took the lead, set fastest lap after fastest lap, pulled out a big lead and then, a couple of laps from the end, the frame-rate dropped again, my lap times dropped dramatically, other cars seemed able to whizz past me even in the hairpin and I ended up in 5th place.
I haven't really investigated this too deeply because when the frame-rate drops the car gets hard to drive and I usually crash but, from memory, this problem always seems to ruin races where I'm doing exceptionally well.
After doing 3 or 4 races on tracks that I know very well I've found that it always seems to happen when the player manages to do unrealistically well.
*EDIT*
I notice that it's possible for players to edit the configuration to remove the "rubber-banding".
I haven't messed about with modding the game at all so all that stuff is still standard.
Perhaps the "solution" is as simple as editing the config file to remove the rubber-banding?