Mystery shifting problem, big mpg drop, out of ideas.

My '94 4.0L automatic began acting up several weeks ago. Both mechanics in town think I'm looney, saying nothing is wrong, that it's just old, high miles (294,000+), etc., but the symptoms started suddenly and are pretty dramatic. Here are the details:

Seems like the transmission takes a really long time to shift. It happens through all the gears, but is most noticeable between 3rd and 4th. RPM's (normal: cruise at 2000, up to 3500 when pushed) now constantly jump up to and hold steady around 4000 when under any accelleration or going uphill until I let off gas pedal, without speeding up or shifting to the next gear. Sometimes even seems to drop down a gear and lose speed with the accellerator floored. Doesn't have problems holding a speed, just getting there. Gas mileage also dropped from near 20 mpg to under 12 mpg when this all began.

Transmission shop says fluid color & level look great, no codes pop up when scanned, and tranny just doesn't shift because computer isn't telling it to shift. Told me it could be a TPS issue or just cold weather. (Weather was still warm, in high 70's, when this started, so cold weather doesn't seem a likely culprit. Also, it does same thing no matter how long it is driven.) Local mechanic changed out the TPS after hearing the symptoms, with absolutely no change in the problem. We did have the water pump replaced a few days before this all started and a broken headlight wire fixed several weeks before that, but no other recent work.

Any suggestions?

Problem resolved. Catalytic converter was plugged, and electrodes were burned off from ALL of the spark plugs. (Not sure how it still ran that way....)

Haven't had it back long enough to see if the mileage is better, but WOW, what an improvement! Driving it is fun again, not simply a necessity.