96 Honda Civic DX Shifting Problems

I have a 1996 Honda Civic DX 2DR four speed automatic transmission.

About 6 months ago I changed the transmission fluid with a simple drain and refill.

About 2 weeks ago the gas mileage dropped from 40mpg to 27mpg. I began listening to the car very closely as I accelerated. I can easily here that it is rarely shifting into the highest hear. It lasts shifts around 35mph (I don't have a tachometer).

Sometimes on the highway it will shift correctly only to shift down a gear after 30 seconds of driving.

Any help would be great, thanks!
cm; Make sure that you used the correct fluid in the trans., just to rule that out. An engine problem can cause transmission shifting errors. Do you have a check engine or service engine soon lamp illuminated? Did you change the trans. fluid because of this problem? Does the speedometer operate correctly? No glitches, bouncing needle, ect?