Engine power loss

Hi,
the car is getting slower. There seems to be a loss of power between 2000 and 4500 RPM.
Starting from a stop, I would start right away and fairly quicky, and when the second gear kicks in, the engine has to rev up to 4000 RPM to keep up with traffic.
From 50mph to 60 mph same thing, I push on the gas and there is no response unless I push harder and the gear changes, gets to 4000 rpm to gain some speed.
The transmissions has been rebuilt. It did this before my transmission died.
I use a lot of oil. I had the exhaust checked for back pressure and the exhaust is ok. So, it does not seem to be a back pressure problem.
There is oil in the plugs area, the gasket has been changed 3 times already and it still leaks.
A PCV valve has been replaced next to the transmission.
There is no visible smoke coming out of the car, so I don't think it is the piston rings.
The air filter is faily new and a new one does not improve performance.
Somone suggested to have a gas line pressure check, could be lack of gas, but at 4500 RPM, seems like the car takes off and regains its power.
Cas has 153200 Miles, 1997, engine 2.0 automatic, and has been doing this for at leasts 60 000 miles.

Any idea on what can cause this lack of power in that RPM range?:banghead:
Thank you.