Always starts fine in the morning from cold, however when warm, it has a problem starting at least once a day. About 4 out of 5 warm starts are ok. On the problem start, it turns over ok (starter motor sounds good and strong) but either won't start at all or runs very very rough and shakes for about 5 seconds and then dies. Once like this it won't start despite repeated attempts. However I have subsequently discovered that if parked on a slope, and I roll forward, try the start and drop the clutch it will start and run fine! In desperation I have even started it on level ground using the starter motor to get a bit of momentum. Next morning starts fine again. Anyone have any suggestions as to what might cause this peculiar behaviour?