96 riv supercharged tranny downshifts in cruise on flat road

96 supercharged riv with 150k, when going down interstate with cruise locked in at 75mph, the tranny will down shift for a second or two, then upshift, does it 2-3 times in 40 miles, no ses light, changed tranny fluid and filter, both looked good, no metal shavings in pan. any ideas appreciated.
thanks in advance

I know you said flat road, but is it really flat? a slight incline could(stress on "could") cause a downshift if its a long slight uphill. In most cases 4th gear should pe powerful enough to do moderate pulling up a slight hill.

During all of this, does the car maintain 75, or does the speed drop to say 65, then the car realizes, oh crap! and then tries to get back to 75? That could be indicative of a bad cruise control unit.

Your problem is somewhat hard to diagnose because the cruise is factored into the equation. The best question to ask would be, does it do it when you don't use the cruise at 75? i.e. can you maintain 75 on the freeway with normal acceleration and not have the car downshift? If you have an intermittent contact in your brake switch mechanism your tranny could be forced to downshift but that should also kick the cruise control out... and a bad servo may throw error codes, but will not light the SES light. You may want to have your codes read if you suspect the servo unit.

thanks for all the prompt replies, I know this is hard to diagnose, that is why I posted it, just to get more ideas, the car maintains 75 when it downshifts, you can definitely tell there is not much load on the car to justify downshifting, tried it with out being in cruise, and it did the same thing, it doesn't always do it, just once in a while to make you wonder how many more miles are left in the tranny. Any other ideas appreciated and thank you all for your time and suggestions.

how much does the RPM's change when the car "downshifts"?