I also have a 95 3.slooooooow and have had the same symptoms, sluggish idle dipping between 250 and 750 but with a check engine light. It also goes away for me rather quickly. I did the diagnostic thingy and got 21 (O2 signal). Have not had the tine to replace the sensor yet. Hope that helps...
Well I have been driving my 95 3.0 for about 2 years and city only driving, I average about 11 MPG. When I can finally get on the highway i get around 18 MPG.