2000 GP trans problem....help?

I have a 2000 Pontiac GP GTbought new in Sep 2000, it now has 80,000 miles..........mainly interstate/highway driven and now after 30 mins of driving or hot days my tranny starts to whine and shift erratically.................had the trans serviced and tested in Feb, the fluid was dirty when changed, not sure if flushed or drained and the test results were inconclusive but i still have the same problem.

Any suggestions?

I have a 2000 GTP and it was whinning between shifts at 56k. Shifting was OK. The whine got worse and the dealer ended up replacing the trans under warranty. All it said on the receipt was "Tore down trans, metal shavings throughout. R/R with factory rebuilt trans."