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.

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