2000 LS2 Tranmission (not) Shifting @ high RPM

Hi,
My 2000 LS2 shifts fine at normal/light acceleration, but when accelerating hard - taching up to 4k or higher - the transmission won't shift out of 1st. When I let up on the gas at that RPM, it eventually shifts. This occurs from 1st to 2nd, and 2nd to 3rd. Probably only those gears because by 3rd I'm already at 60-70mph. Fuel pump pressure is good and I just changed the fuel filter. 59k miles on the car. Gas mileage ok: 20-21 around town - haven't taken it on a long trip yet.

Any suggestions?

Thanks in advance.

John

For anyone interested..., the problem was solved when the water pump went out and the tech discovered the timing had shifted. During dis-assemble, he observed alignment marks were not lined up where they were supposed to be - they were off approximately two sprokets. When all the new parts were installed and lined up per Saturn specs, the car now runs like it should have all along. Hard to tell how this happened in the first place but now, the engine runs great. Mystery solved.

Saturnjohn, My L-200 is doing the exact same thing now. Please tell me more detail about this "shifting timing alignment marks" & "two sprockets off"