I'm riding fixed with an profile imperial chainwheel, not sure if that matters. But I recently changed my wheelset and I have been experiencing an issue. When I put a good amount of the pressure on the pedals, and they would frequently just give way and I almost lose balance because of my legs suddenly jerking in the way that they were pushing the pedals. The crankarms would keep spinning but I wouldn't move. Can anyone tell me what the issue is?