1996 240sx, AT, shifting acting suspicious

[chato de shamrock]

I drive a 96, automatic, all stock under the hood.

Hey guys. Lately I'm noticing my car doing something with its gears and causing the engine to sound revved up more then I think it should. A few weeks ago I was driving on the freeway when I noticed the car seemed to down shift and at the speed i was going (about 70mph) the engine sounded like it was being foced too hard. I let go of the gas pedal for a few seconds and then pressed it and the car seemed fine afterwards.

Some time last week I was driving to work (about 35-40mph) when it did the same thing. The car seemed to down shift 1 gear and the engine seemed it was being forced. All these times it happened it caught me off guard and I wasnt sure what was going on.

This past Tuesday I was entering the freeway on an uphill onramp. The car seemed strong on the 1st gear, 2nd, 3rd gear. By then I was already in the freeway, but it was an uphill road so I kept raising my speed. Everything was fine and the car felt strong but listening to the engine you could hear it reached the point where it had to shift into 4th gear. The engine seemed to be revved up as much as it could on 3rd gear. I let go of the gas and let it cruise then stepped on the gad and sped up some more to see if it would hit 4th gear, but it wouldnt. At this point I knew it was doing this again so I checked my RPMs and noticed they were just over 4000 and I was driving between 61-62MPH. I let go of the gas pedal so it cruise on the speed. Doing this helped in the past but not this time. Once the road started going downhill I cruise and gently pressed the gas pedal again. It still seemed to be revved up high so I exited the freeway and stopped at the offramp red light. Once the green light went on I drove it on the road and everything was fine. I got back on the freeway and drove from around Dodgers Stadium to El Monte and back, everything was fine.

Today I got on the freeway and noticed I was driving 70 at about 2500 RPMs. Big difference from last week's 4000RPMs at 61-62mph. So far this has happened about 5, 6, 7 times within the last month and a half, or so.

The reason why my engine is stock is because I know very little about fixing or upgrading things under the hood, but i'm trying to learn as i go. Can someone point out what may be happening with my car or what I can do to diagnose the problem and go from there?

I thought it might need a tune up but i've driven a car that was well overdue for a tune up and its reaction in comparason to what my 240 is doing are very different. I figure that if it needed a tune up I'd lose power at the start and not while i'm in the middle of driving in 3rd or 4th gear, am I right?

As always, thanks for your help guys.