My Jeep runs on both Petrol and LPG. Sometimes she fires up fine on either fuel and purrs like a kitten. Turn her off and she will just not start. Leave it for a few hours and she starts again. So frustrating. Twice had to be recovered. She has new plugs, ht leads, distributor cap and rotor arm. Other times she fires starts and the the revs just die off and she stalls. The petrol pump is priming. Using a spare spark plug checked for spark. Whilst she is firing before she stalls, lovely blue spark. When she stops firing there is still a spark but weak and very intermittent.

Help please and thanx in advance.