Crank slipping

Errm, that was my thought too.
I've checked (and reassembled) all I can but still no difference, which is why I asked the question. Can right crank arm come loose from spider?

Stumped :(

what style cranks are they? if they're one piece forged crank and spider if the 2 aren't one piece you're probably fscked already. check for cracks.

the only possibility of them not getting along are those cheapo cranks with forged crank and a tin foil chainring/spider attached.