I have a E150 Ford Custom Van and can't seem to get it tuned up?

Went to my mechanic to have a tune up it was missing for a week. He replaced, all air filters, gas filter, belt, spark plugs, cap, rotor, wires. It did great for about a week and then it went back to missing but not as bad tho. So he replaced the coil but a used coil. It was fine for a couple of days then it went back to missing. This missing really only happens when your going up a hill at a slow rate of speed. When your driving on the interstate it can do 75 with ease, and does not miss. When the van is cold doesn't seem to miss when starting off in the morning. Van is sluggish all the time. Help?

Added (1). He also replace pcv valve and adjust timing.

Put a brand new factory coil. No joy, have a compression test done on the van.

Putting a used coil on the van was a mistake. Parts that are subject to deterioration and wear should always be replaced with new parts.