I always get one (or two) episodes of hyper-stuttering/freeze for 2 seconds each, usually in the first 5 minutes I am in the server or so. This happens every time on your servers (incl.. XGN practice) but not usually on the race nights server. It does not do it offline and it does not repeat after those first few minutes. I have no idea of what it might be.
Granted: my system is pushed a bit to the limit. It is normally fully stable at 59.999fps, but turn in at T1 of Barcelona gives me a little bit of stuttering in some cars any... don't know, 10 laps (some other corners/cars have similar issues, but even less frequently). But these online freezes are of a different type: they are longer and more freeze-for-one-second than stuttering.
It is somehow linked to multiplayer, but after all this time I have not managed to figure out what it is. But given that it happens only in the first 5 minutes or so after I join, I doubt it is linked to other players joining and mr.