tps or other problem

[SVXPILOT]

For the last 50'000 miles my SVX has wanted to downshift to the next lower gear for no reason. always when cruising at speeds up to 80mph with light pressure on throttle or cruise set. when in drive it will drop into 3rd than right back to drive. If I have it in 3rd while driving slower it will jump to 2nd and than back to 3rd. when in 3rd I can press manual button and it will stay in 3rd. above 80mph in drive it doesn't act up and at higher speeds in third it doesn't act up. Some times it is much more active than others.

I remember reading that someone on this site had same problem and the tps was at fault.

what are the symptoms of a faulty tps?

Other than the above problem trans is fine with firm quick shifts normal fluid color and I change fluid every 10'000 miles when I do my oil changes.