Car losing power after it warms up.

I am trying to help fix my girlfriends Geo Metro. It accelerates fine until the engine warms up a bit. Once the car has gone a mile or two it gets sluggish. It slows down and has a very hard time speeding up, seems to be working way too hard once that happens. It also appears to be going through coolant at an alarming rate though the temperature gauge remains on the low end of normal. Once the car has sat for a bit it runs fine again until it warms up.