95 GC transmission probs

I have a 95 GC w/ I6, after the engine and transmission warms up and I get to highway speed the revs will cycle between 2,000 to 2,100 rpm (even on cruise), then randomly the trans will shift into 3rd and run at 3,000rpm. This does not seem to be load dependent, it will down shift on a fairly steep hill while I'm backing off the throttle. This transmission has about 20,000 mile (rebuilt). The shop couldn't find anything wrong. They asked about having another shop check it out the engine and transmission codes I agreed. Well nothing other than "high secondary ignition resistance". I tuned it up plugs, wires, cap and rotor, it was also suggested I have them perform "fuel injection system flush". The transmission shop "is happy to fix it under warr" but doens't think the problem is in the transmission and claim the computer is telling the transmission to down shift? While this could be true downhill with no load makes this hard for me to swallow. Any insight or direction would be appreciated.