I used to have one of these with the same type of problem. The garage I bought it from replaced the head gasket, injection system, ECU, and loads of other stuff, and it still did it.
In the end it was traced to low fuel delivery pressure. A faulty fuel pump. This was replaced, and all was well (for 2 years until the head cracked)
Some words of warning. NEVER use radweld in this engine. Always keep the right amount of antifreeze in the radiator. Flush the radiator at every service. Check your water level weekly. If the temperature gauge hits the red, stop.
Thes engines are great, but will overheat and cook very quickly, if you look after the cooling system, you will be fine.
Good Luck
K Ferris, September 2004