engine misfire

Just purchased a 2000 v8 with 160k miles,and it is driving me mad ! Ran fine at first ,then developed a misfire on starting in the mornings.
Idle control valve tested and found to be sticking so replaced, maf sensor giving low voltage readings so replaced. ECU was showing intermittent faulty readings on lambda sensors (voltage less than 20mV) and this would vary between left and right hand bank or both. Wiring checked between lambda sensors and ECU and no faults found. Replacement lambda sensors fitted (type with universal connectors as no-one had stock of the sensors with the proper connector at the time). Problem still present so ECU changed. Car ran for a couple of days then misfire returned and car using a small amount of water.

Performed head leakage test and exhaust gases were detected in the header tank. Compression test showed low compression on no 2 cylinder. Found head gasket gone due to faulty fitting. Changed the heads, fitted new head gasket, and rebuilt upper part of engine with new coil packs, plugs, plug leads, etc whilst we had access. The engine ran fine for 10 miles, then the misfire returned. Refitted lambda sensors with new connectors and engine ran fine. Ran car for three days for 50 miles with no problems, in town and motorway, but now it starts and immediately runs rough, misfires and then stops as if its run out of fuel, but will restart for a few seconds if I turn the key.
Can anybody suggest what is causing this problem as I have run out of ideas.

Fuel pump.

re:-engine misfire
Am purchasing a fuel pressure tester ,as I am fed up just adding parts to this ***** vehicle.
Ret.h

Re:-Engine misfire.

Tested fuel pressure today-50-52lb/in2,so it cannot be fuel pump ! Any other ideas please.