hot engine hard to start

Just had transmission rebuilt and new problem has occurred may be a coincidence, not sure. After driving approximately ten to thirty miles, oil pressure and temperature normal, no loss of either fluids, but attempted to start the van-turned over slow, started but shut off next attempt one slow crank did not start. Got battery jump drove to auto store and bought top line battery both old battery solenoid alternator and starter tested okay. Also wrapped starter with reflecting metal to keep starter cooler. Today after thirty mile trip, shut off engine for approximately ten minutes. Started slow, shut off, next turnover was slow almost nothing but it did start. I have used a head gasket sealer, no visible fluid loss of discoloration-seems like there is back pressure in the cylinder(s) when engines hot causing hard start. comments? John