1997 Ford Escort 5th gear acceleration hiccups

Hi, I own a '97 ford escort which is getting up in mileage. I commute roughly 90 miles a day, and have been recently experiencing trouble when in 5th gear and accelerating past 50 mph. It seems that when I keep my foot on the accelerator at a constant rate or try to accelerate for any period of time, the engine feels as though it hiccups or almost as though it has a momentary lapse where it may stall out, but doesn't stall. I also noticed that once into the 5th gear, my speedometer is useless and bobs up and down never giving me an accurate speed reading. I tried replacing the spark plugs and wires, but that didn't seem to help. The engine in the car was replaced roughly 75,000 miles ago. Any suggestions or ideas on what I should try next?