My instinct tells me that it's a fuel related problem. Can you hear the fuel pump when you're trying to crank? I had a similiar problem once on a 82 model 240 and it was caused by the fuel pump relay. The contacts were burned and sometimes it would pass enough electrical current to run the pumps, and sometimes it wouldn't. Also water in the fuel can cause similiar problems. Have you replaced the fuel filter lately? You'd be suprised at how much water these filters can trap and store (it won't pass thru the filter). Also clean the fuse contacts that supply the fuel pumps. Again, it sounds like a fuel pressure/flow problem to me.
steve