Engine performance suffers after fully warmed up?

Hi -

I have a 92 Honda Accord EX with approx. 140,000 miles. Motor sputters - most noticeably at idle, then gets less noticeable as RPM's climb. Obviously this negatively affects performance in general but curiously, as the car warms up, performance gets worse. Once the motor hits normal operating temperature, car starts bogging down badly at low RPM's. Only when revs pass 3000 RPM's does the motor seem to "wake up", but even then there is still a noticeable lack of power. Changed the spark plugs/wires but there was no noticeable improvement. Paid $100 for fuel system cleaning at auto shop and there was a small improvement, however, all the symptoms are still present just not quite as severe. Any advice is much appreciated!