Foamy is not good. I would monitor the plugs/cyclinders for a few start attempts to see if the "foamy" re-appears. Compression is good... so head gasket is ruled out.
Some 750's have a crankcase drain. There is a "choke-like" knob on the engine or exhaust pipe. It opens a valve low on the engine (below the exhaust). If you have one, wait till you have a "no Start" problem, then crank the engine while pulling the knob, and look for water exiting the drain valve. Incidentally, these valves are known to fail, and create a crankcase leak... many owners remove the valve, and install a block-off plate.
Maybe it's not water at all. Maybe its just a hot-start issue. Weak stator windings, or ignition coils get weak or fail when warm. Then work better after cooling off. A weak coil might work well enough to keep running at normal engine speed, but fail to fire the plugs at cranking speeds when hot.
Anyway, I'm just guessing at this point. Good luck.