I posted this earlier this evening for your question on another site:
I had a similar problem on our 2002 Class C with Ford V10. The engine run great except under a light load, usually going up hill, even slight, and in a higher gear.
To me it seemed like the engine was shuttering or slightly missing. One on level ground, highway speeds, etc. it ran normally. I searched this forum for a solution:
Ford Truck Enthusiasts Forums - Ford truck and SUV owners and enthusiasts Community And Information Source.
From searching here it appeared the problem was likely a bad coil over plug (COP).
Some suggested if one was weak, others could follow and go weak. Ford wants a bit over $100/each. Most suggest buying them else where. Someone suggested a
seller on Ebay selling sets of 10 for $140 complete with boots,etc.& free shipping.
I got them, replaced all 10 and the engine ran fine after that . This was 3 to 4 years ago. Some also listed other suppliers & part numbers but check first :
Motorcraft DG508 & DG491
Ford F7TU 12A366 CD; F7TU 12A366 AA; 1L2Z 12029 AA; 1L2U 12A366 AA; 3W7Z-12029 AA
Delco F523
Standard/Hygrade FD503 & FD493
Tru-Tech FD503T
SMP/Bluestreak SPP39
Wells C1139
AIRTEX 5C1127 & 5C1412
BORG WARNER E213 & E262
Here's an example from Ebay currently:
http://www.ebay.ca/itm/FORD-4-6L-5-4...a7b233&vxp=mtr
Good luck solving your problem. If you change plugs, be carefull, spark plugs torques and procedure is important on this engine.
