Transmission problem

1997 Mazda protege 81K miles, auto transmission. having transmission shifting problems (or so I suspect). there are 2 different scenarios

1) happens always during highway driving: transition from 4th to 5th occurs anywhere between 45-55mph. once cruising at ~60mph (rpm is ~2.5K) , if I want to accelerate even a little tranny seems to downshift to 3rd (rpm goes to 3k +) and then returns to 4th quickly (rpm~2.75k). Is this normal or does it mean transmission or some sensor is being super sensitive (or I have a heavy foot!)

2) occasionally, again highway driving, tranny wont shift to to 4th even when I reached cruising speed (60-65mph) and backed off from gas enough. Then I back off to almost no pressure on gas medal, rpm seems to return to what it should be during cruising at that speed, but I end up slowing down and have to press gas and the cycle continues.

At the suggestion of a few here, I checked transmission fluid (changed 22K miles ago) and seemed good color, no particles. I dont think my mechanic (not really a transmission expert!) cannot see the second problem, andthinks its my driving habits. So I observed more carefully : last couple of times it happened, I noticed I had accelerated hard to merge on the highway (i have to do it sometimes because my old clunker does not do a good job of it unless its a flat or a down sloping surface.

any suggestions. thanks

do you know if your transmission is controlled by a computer or is it controlled the old fashioned way( with a valve body)?

5th gear??? Automatic trans.

I dont know if it is. But id does have a computer. does that mean computer must be controlling it ?
(I know theres a computer because when I bought it, the mechanic who checked it out said the computer did not have any codes stored on it.)