no power

i have a 97 gtp....while driving at a constant speed of say 45mph and higher, once i try to pick up speed, it gets really "doggy" the boost picks up but i dont gain speed unless i floor it! the best way i can describe this is to compare it to driving a manual 35mph in 5th gear trying to pick up speed......it just doesnt work:) it still picks up great at take-off, just once you level off and try to gain again is when i have problems........
any suggestions?
Thanks!

you could have a sticky TCC solenoid that isn't unlocking the torque converter quickly enough.

Or a partial failing TPS. Get it up to speed. Let it lock the converter, then step on the brake slightly. Just enought to see the rpms come up. If they do instantly, My bet is that the TCC solonoid is fine. After the rpms go up, let go of he brake and hit the gas. See if this make it accellerate like normal. If it does, i would suspect a bad spot on the tps, and when you floor it, the pcm isnt seeing that and ist unlocking the torqu converter.

im gonna say a TPS to, deff cause the prob your explaining.