should I give them more money??

I had posted a while back about my 99 4.3L S10 mis-firing at highway speeds. I have replaced the plugs, plug wires, and filters. I have taken it to multiple mechanics and the dealership, the end result was a lot of money spent but no one knows whats wrong. It seems to buck the worse when its warmer outside (I didn't have too many problems during the cold January). The dealership mechanic had recommended a fuel system flush. I didn't let the dealership do it right then because I wasn't sure that was going to do any good and I still have no idea how much that would cost. Does anyone know if that flush is any better than running a couple bottles of STP through my truck? Is it worth it?
Thanks

Well I don't know if a flush would help.
I'm fond of Techron though on a regular basis.
No DTC codes?
You don't have a cold air kit do you?

No codes are showing.
No cold air kits.
I'm really frustrated here, I have no idea what the problem could be and neither does any of the mechanics I talked to . they all say that if it gets worse then maybe they can find the source, I'm afraid it will be too late by that time.

I suppose there are a lot of things it could be, perhaps too much EGR flow but I would think that would throw a code.

Was the dealer able to verify the condition? Or, has it run fine for them (yes vehicles sometimes only run great when a tech is in it).

I know Ford dealers have a "flight recorder" that they can send you away with and if it acts up while you are driving it the recorder takes a snapshot of the entire system when you press a button, helps catch intermitten problems. It's an expensive tool and if they have a simmilar device they're not going to offer it unless they know you ( bought it there, not an ***hole, etc)

Change fuels, brand and grade.

Well a lot of people have asked me if I ever got this problem fixed because they have smilar symptoms. Yesterday I replaced the throtle position sensor and had an upper engine flush and it seems to have fixed the problem (knock on wood). I don't know if this helps the others out there with the same problem.