Ford Repair: Fuel system, new distributor cap, ford e150

Question
I have a 1992 Ford E150 conversion van with a fuel injection 351 under the hood. I've changed the gas line filter twice,put in a new distributor cap & rotor, used injector cleaner & valve cleaner.
Problem:When cruising down the road the motor acts like it is not getting enough gas.. like it pauses for a split second then continues. This is most noticible at speeds between 35 to 55 mph. At 65 plus the problem almost disappears.. Why & how do I correct this?  

Answer
Hello and thanks for the question...I'll try to help.
I would suggest a few things in this situation:
1: rent a fuel injection pressure gauge. Install it and look at the reading you are getting.
2: Have the computer system checked for fault codes. Fix those first!
Hope this helps,
Erik