DBCherry is right on, if you have any bubbles in the fuel line, you need to correct it. Repalcing the fule lines is a start. Checking the o-rings are another good area.

Cold running engine could be the culprit as well. A hotter glow plug may be the ticket.

Another thing I would reccomend though is to buy a NEW gallon of fuel and try it. I have heard stories of fuel going bad after it has been exposed to long. An example of this was from a friend having the same problem. But what he was doing was bringing the fuel to the field, then right into his house. The outside temp was around 30 degrees, inside was about 75. This cause the air to heat up and let some condensation into the gas, or at least it is what I speculted to. He bought a new gallon of gas, had to retune the engine due to the tuning measures he was attempting with the bad fuel, and the engine purred!

Summed up: Check fuel lines for air leaks (replacing may be a cheap measure to this,) change to hotter runnning glow plug, and getting new gallon of fuel would all be areas to check into first.

Just my thoughts,

Reg