Toyota Repair: Engine goes into overdrive for no apparnt reason, toyota 4runner, throttle cables

Question
QUESTION: I have a issue with my 94 Toyota 4runner, when it gets up in 5th gear over 70mph the engine revs up every few minutes. It sounds like it is going into overdrive, and this just started to happen. The RPM doesn't go up at all, which is strange.  It's at about 150,000 miles, changed oil, oil filter, plugs (those new bosh duel point plugs), it has new exhaust (glass pack), new clutch, distributor, fuel filter etc. It seem to start to happen when the weather got warmer. Is this a symptom of the o2 sensor? Or something else?
Its not the cruise control, disconnect it and still had same issue.  I think it has something to do with the overdrive, because it revs up like its going into overdrive.. but the engine is not being overworked i.e. going up hill or jack rabbit start.  Todays I used WD-40 on the throttle cables, but still having the same issue.  What else do you think it can be?

ANSWER: Is this an automatic or manual transmission? Sorry, but I'm a little confused, only the manual transmission has 5 speeds, the automatic has 3 speeds with overdrive, htere are two statements which are contraditing, "the engine revs up every few minutes",and "the rpm doesn't go up at all" if the engine revs up the rpm has to be increasing, please clarify.

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QUESTION: 5 speed, its a stick.  RPM, is the RPM Gauge, this doesn't go up but the engine revs up.. like its going into overdrive.

Overdrive might not be the right terminology, but the engine revs up as if it were going into overdrive.  I wonder if it could be one of those Bosh duel point spark plugs going bad>? I'm lost here, I think I've tried it all.

Answer
I'm still not sure what the problem may be because if the engine revs up the reading of rpms on the tach should go up, if a transmission goes into overdrive the rpms drop, based on the information I have so far it sounds like there may be a fuel pressure loss or possibly an engine misfire, try using the factory OEM plugs, is the engine timing set correctly? Was this problem there before all the work was done?