My experience has been that they gradually decline, unless they are hit with moisture or something foreign in the exhaust. On my '97 I had not had any problems with my O2's until I did the seafoam thing, and then I lost both the upstream sensors in just a few weeks. I think the '95 sensors are different, I believe 95's are OBDI, not OBDII, so I don't think the O2's are heated on the 95. It has been my experience that the non-heated sensors are more robust and will take a little more crap before they die. In that case, I would go with gradual failure. The rough idle has probably been getting progressively worse for a long time and now just got to the point that you are noticing it more. My DD is an 88 Isuzu, and that is how the problems with it are. Now that I have put close to 100K on it (305K total), I am starting to notice a few of the little reoccurring problems sooner. I am getting more in tune with the rig. I think that is inevitable with any 300k rig. Sorry for the birdwalk, but in short, yes I think they decline gradually.