erratic engine power

I have a 1995 honda civic (automatic) and it?s in great shape and normally runs perfectly. Sometimes the engine pulls smoothly and has no problem getting me up to high speeds and staying there (especially after long periods of driving). At other times it feels like the car is struggling just to go 70 and I have to hold the accelerator hard. Occasionally it will be struggling and then for a brief period it will seem to run smoothly again. Also, when I accelerate there seems to be a surge of power around 4,000 rpm. This is a non v-tec engine so it isn?t that causing the surge. What could be causing this behavior??