Andrew,

Since you have replaced the regulator and diaphragm, I assume you have some knowledge of the LPG system. Do you see any of the lines or components getting frosty? Make sure coolant level is correct. Any codes or lights showing on dash? There is a plug that you can jump the A pin to the D pin, turn on switch, do not start and watch the engine light on the dash. This should flash 3 times 112, after that additional numbers would be problems. The plug is located under the floor board on the left hand side. How long would it need to sit before starting? Could have a fuel valve sticking or possible contamination. Since it only happens at operating temp, code for codes then. Separate the mixer from the throttle and check for buildup. I would say something on the fuel side is sticking when you shut down as it takes a small amount of ether to start. Careful with that stuff.