fix

So I've been riding my fixie around and recently it's developed some odd problem that I can't diagnose. I built the damn thing myself, so you'd think I could find the problem, but nooo.

When I go hard forward, like out of a green light, the pedals slip forward about 15 degrees. They're fine until I do the same thing in reverse, skip-stopping or even just a hard stop. Then they slip 15 degrees backward.

I tightened the lockring and I swear that's not the problem. My left crank arm is losing life quickly, but that doesn't seem like it's the problem in this case either. Any ideas? I want to ride without fearing for my life.