i had the same problem and one of my friends told me it was to do with the driver and when you kik it up and hold your seat does cranks turn round on its own
If you're chian is too lose your cranks will spin, tighten it up a bit and try, then tighten it up some more until they don't spin. You only want like a little less than a quarter inch deflection of your chain otherwise it's too tight.
Do what can mccaul does, drill a hole through your frame under your bottom bracket and put a blur brass bolt through it. It rubs on the crank axle and wears it down but it is a way of doing it so yeah. Peace