4 Wheel Drive/SUVs: 1998 Toyota 4Runner SR5, toyota 4runner sr5, changing spark plugs

Question
There are 213,000+ miles on my 1998 4Runner SR5, and for the past 3000 or so I have been battling a bucking, misfire-like problem.  Having consulted my local shop, it not coding, changing spark plugs and wires then moving on to coils and replacing them one by one, it is still a problem.  What happens is, after running the Interstate for an hour, sometimes more, a very noticeable power loss results, and a bucking, misfiring feeling occurs.  When allowing it time to cool down, then continuing on, the problem does not again arise until heated (no overheating has ever occurred).  In and around town the bucking is not noticeable even on sustained uphill acceleration.  What do you think I could be dealing with?  Thanks for your time.

Answer
J, I would have them check the fuel pressure while sitting there and while going down the road, also the fuel filter, Other than that, it would be hard to diagnose without looking at it , sorry it took awhile for a response, I have been out of town ADAM