After about 10mn of intense SMB action I always have a CPU burst which causes the game to heavily lag. It occurs every time. I have to quit SMB & Steam and reload them to get rid of it but it doesn't work all the time.
I have a pretty old config (AMD Athlon 3700+/1.5Go RAM/7600GS Video card) but this game used to run smoothly on this PC before last updates. I can't precisely tell which update could cause this because I didn't play SMB for a while and there might have been 2 or 3 updates inbetween...
I also tried to deactivate vertical sync on my Video card panel but it doesn't change anything.