low gas mileage

The gas mileage on my 99 Trooper is way down. I drive about 60 mi round trip to work 4 days a week on an interstate so mileage should be at the Trooper's best - not that great but not bad either - 19 mpg. It's down to 12 or 13 mpg. What can be the problem? ERG valve has been cleaned, some gaskets were replaced, no change. Any suggestions? I like my car but this is a really high cost drive these days.

Perhaps your O2 sensors are going bad? I also just read another post from somebody that replaced their cats and their mileage went back up. My 99 is getting 15.5 right now w/ AC running. Seems a little low, but after reading about other troopers getting in the low 20's, I'm considering replacing the cats also.

Also a leaky intake manifold gasket is also a common cause.