1995 unknown problem

I drive a 1995 Olds Aurora, i have 201000 miles on it.

A couple weeks ago it was working fine, then all of the sudden its R.P.M. dropped from 2100, to approx 1400. It doesn't sound like the car is shifting, but the rpms still drop quite a bit. Also when the problem happens, my car vibrates a lot, when i listen it sounds like the vibration is coming from under the back seat. My gas milage has also dropped a considerable ammount since this started. I brought it into a shop they hooked it up to their computers, and they couldn't find anything wrong with it. Has anyone had a similar problem, or know what might be wrong?

Welcome to AF. It looses power even at throttle? If so possible bad cat or low fuel pressure What speed/conditions does this happen? Does it feel like an engine misfire, your description is sketchy.

it doesnt seem like a misfire. At idle it does run pretty rough, and it happens around 45-50 miles per hour.

You need to determine the running rough at idle, this translates into a loss of power and shudder under load, there are too many possibilities to address them here, maybe you should have a pro check this out for an accurate on site diagnosis.

no check engine light?

The check engine light is not on..

I'm bringing it to another shop on friday to have them take a look at it (more problems have developed), from my discription, he says it is possible an ignition problem, or a loose vacuum somewhere. hopefully he finds out what it is.... and its a cheap fix haha.