2000 Grand Am SE

Was driving on the highway and all of a sudden my mph gauge just drops and my car slows down. This is the first time it happened and it didn't do it again after I got off the highway. Other things it has been doing: eating gas, shifting hard, and doesn't want to get up to speed. Not sure if they are related to this new problem or not. Any ideas? Someone please tell me it's a cheap fix! I just paid Midas $800 a month ago to fix everything that was wrong with it :banghead:

4 or 6 cylinder?

Exactly what did Midas do for $800?

4 cylinder.. Brakes, brake pads, serpentine belt and pully, oil change, spark plugs, two diagnostic tests... Just a bunch of miscellaneous stuff. I wish I kept the receipt to give you a full list :(