I have an early 70's Peugeot U0-8 which is now a fixed gear, and a very noticeable problem that has been bothering lately has to do with speeding up and slowing down. Whenever I try to accelerate suddenly its as if there is a grinding/rumbling feeling in the bottom bracket. For the record, I replaced my cottered cranks with a newer crankset that attaches via the square tapered spindle. I also overhauled the bottom bracket. I also had to acquire another fixed cup, as mine was shot. I had to replace my chain as well, and I'm using an 1/8" chain. I don't recall having this problem before I replaced everything, so it must be something I've caused. I know its not the chain tension either, because the tension is near perfect. Sorry for my lack of technical names, I'm "new" to bikes. Anyone have ideas?