freewheel

Ive been trying to solve this problem for a while, but haven't made any progress. My 98 Legacy hesitates while accelerating until it reaches 2500 rpm, then the power surges and it accelerates fine. Always 2500 rpm, and in any gear. Ive replaced the plugs, wires and added a K+N filter. I had a mechanic pull the codes and it indicated a bad O2 sensor, so at 72,000 miles I replaced both. But the problem persists and now is getting worse. Ive read here that the MAF sensor could be suspect, but no CEL light or codes indicating such. Today I disconnected it and started the engine as per a previous post, and it idled fine but revved very poorly. Can anybody shed some light on this problem or at least point me in the right direction? Thanks