1994 Ford E-Series Van loss of power

On the open road the van seems to do fine with an occonial miss changing from 3 to overdrive, how ever when I go up hills it begans to miss some and when I get to my steep drive way 7% grade that is 500 feet long it miss so bad that I must shift into first and it creeps up barly making it and sounds like its running on 4 cylinders and almost stops, the only way I can get it up is to press the petal and than let up over and over again, when it finally does reaches the top the engine quits missing and all seems fine again. I know some about cars as I have worked on a lot of diffrent kinds, I changed the plugs and wires and roter and cap which dident change the problem, and it does not seem like a pluged fuel filter, but more along the lines of a vacume advance problem. I am not fimular with this since is not like the old type that have a vacume tube running to the dist. I am assuming that this has a electronic advance or somthing else is wrong. Possably a timming chain that is going bad? Can you help?