I know it's a late reply to your question, but I recently encountered the same problem with a clients S55XM Hyster with a GM 3.0L lpg engine.
Turns out the engines had a problem with braking the starter pinion or even the nose cone of the starter when the engine fired immediately when the operator began cranking over the engine, so they incorporated a start delay electronic module just to the left of the battery, it would randomly fail and then start working again,, we removed it and installed a 12 volt DC delay timer we purchased for around $25 bucks from Fact#rymation, set it for about a 3 to 5 second delay & have hade no problems since... Much cheaper than the $1000.00 OEM
Hope this helps someone somewhere....

thank you for responding. we have checked wiring previously

Keyswitch and supply fuse, check input voltage to primary side of coil. if voltage is low trace it back to the poor connection. I had this happen wehere someone had replaced the fusible link on a truck with a 30A fuse and cheap holder, and corrosion on the output blade melted the fuse and holder so it only fed the coil intermittently.
Wiring is where I would have started, not ended... a distributor could have bought and paid for a nice FLUKE meter...

new lpg carb was installed also.

fuel supply is good. unit may run for 8 hrs. or 1.5 hrs. then quit

Check your fuel supply to carb