Now I wonder what the cost of that lot was?.
Seeing that you have already changed the rocker covers, the chances of them all being warped is rather remote I should think, the rubber gasket is capable of taking up a fair amount of distortion, so i would rule that out.
The head gasket would in most instances give you other clues to go on, if it has gone across a waterway it would pressurise your coolant, similarly if it has gone across an oilway. Difficult starting, oil out the back of the exhaust, steam out the back etc, back firing, lousy running. If you can rule those out it aint your head gasket. but it would not improve with revs in fact the opposite.
Your rocker gasket leaking would not give you bad running, just a mess, but then it would depend on what it was leaking onto.
You have changed the dis pack and plugs, why?
I would be looking at the icv and accelerator, fuel filter and pump, injectors.
The moral of the rocker cover gaskets genuine parts or aftermarket, they both are capable of failure, just that 1 is a lot more expensive than the other but good for the bottom line. The use of the adhesive to me this stuff will do what it says on the tin "it sticks" and thats about all, it seems to dry quite hard, to hard for my likeing, I use instant gasket and dont change the gasket, never had a problem it stays very flexible and where it is put. But then each to his own.