1st thing - perform a GOOD tune-up ( Plugs, plugwires etc ) and check voltage and voltage drop to ignition system - excessive drop of ignition voltage during cranking will cause a hard start.

2nd - check voltage drop across starter during cranking - you may have a dragging starter causing hard starting.

3rd - 30 min warm-up is WAY too long - replace engine thermostat and I agree with Edward T, install an in-line temperature regulator ( thermostat ) in the OUTPUT water line from the LP Gas vaporizer to engine.

Lets clear up the hard start and stumbling until warm before doing any checking/diagnoses on the hydrostatic drive.

Also include an e-mail and serial number as some of us may have more info availible