1996 Ford Aerostar

Computer problem
1996 Ford Aerostar 6 cyl Two Wheel Drive Automatic 175k miles

Engine for a year has check engine light coming on
when accelerating past 50 uphill or 110 klm's on highway! Starts missing! Been good when on a level surface lower speeds! Engine has now been missing in 3 cylinders! Will not run properly- New coil pack installed- No help! 1996 Aeorstar 3.0L
Spark plugs fouled! Any ideas? Mechanic having trouble finding the problem! Otherwise has been great so far- No leaks or oil consumption!
New engine computer has not changed! One bank still out! Manifold on 1 side checked looks ok ! Engine compression good and cylinders have spark and fuel!
Also 1 year ago fuel filter replaced and problem started shortly after. Never had been changed to 160.000 klms or rather 106.000 miles !
New engine computer tried! Does not help! Thanx Ross
November 18, 2009.

Have you checked fuel pump pressure? Have you checked the catylatic converter?

Let me know.