Only an intuition here, but you might wish to swap out the coil. It has been known to overheat when mounted on the block and to cause starting problems thereby.
The solution (beyond getting a new coil, checking the timing and replacing the wires) is to mount the coil on the bulkhead away from the block. This allows cooler air to keep the coil at a functional temperature.
That said, the layering or cruddy gas is a possibility, and you might wish to check out your tank vent for partial obstruction from insect activity. Spider goo killed my engine once, and it was very frustrating because the engine would die gradually over 10 minutes or so, wouldn't start for five minutes and then would start...only to die again. Switching up to a 5/8th inch vent hose and replacing the screens did the trick.