as mentioned, possible bent pedal or BB is toast. but, as yet to be mentioned, is a problem i, for sure, had that will exhibit the same symptom. a bent or poorly manufactured crank arm. mine was on the non-drive side. i swapped out the bent or possibly non-square pedal threading arm with an old square tapered arm i had and all was well. occasionally i forget they are not a matched pair, but it seems i'm always being reminded of it.