What can it be

My p38 was running real bad lumpy as you like and no power. Took it to a garage and they said start with new plus and leads. Done that and when i picked it up i though wow thats ten times better. Problem is the car still has a power loss now and again it can come and go when it likes but normaly when its under load.Even a slite gradient will be enough and when you have a 4.6 lump doing 40mph its not funny well it is to those going past me.Please help me so my p38 is not the laughing stock of my town. The car is not loosing water and the garage tested it and said the head gasket is ok. Some times it is fine and you would not think it has a problem.

have yoy changed the fuel filter, if yours has got one that is replaceable?

I would agree with al-one-arm, sounds like fuel starvation. check the filter and fuel pump.