I'm suffering from the same problem. It was gradual at first. I noticed I needed more power when i'm going into first, otherwise the motor boggs down and I get closee to stalling. If i'm in 5th going around 40 and I have to decelerate for whatever reason, I can't get back to 40 without either downshifting or putting the petal to the floor( I'm used to getting at least a little more pull all the way through 5). Plugs, wires, and oil may need to be changed but I found something funny: My dipstick smells like fuel. I'm still diagnosing, but its possible it may be piston rings.

I'm having a little trouble with finding how to pull my error codes ( my check engine light came on after about a week and a half of this power problem ), any one know a good site with code descriptions and how to pull them?

Its a '98 GS i've owned for about a year, but it was down for about 3 months while I replaced 4 valves bent by a snapped timing belt. Haven't really done much research or work beyond that and regular maintenance. Its got 111k miles.