I have replaced plugs, leads, coil, distributor chasing a severely missing engine problem. Seems to be fuel-related but I have purchased a pressure test gauge and followed shop manual instructions and all the readings are normal and steady. Sometimes engine will sit and idle normally for a minute or two then suddenly act like it is going to die for 10 seconds then revive only to repeat the process. When driving the thing just misses badly and erratically. Any suggestions gratefully received

 

have you checked out the pistons, and other internals? also have you checked the oil pump?

 

Give it a compression test, any cylinder with a test more than 20% less than the others is a sure sign of the need of rings, piston, or valve fixing.

 

Have you narrowed it down to one cylinder and have you checked the injectors? My guess is that if the truck runs good while it is in neutral when it is using the least gas is that it may be a clogged injector and would advise that you try some fuel injector cleaner.