Although you don't really mention in detail the problem or the engine, I'll share my similar experience with my 94 4.3 4dr s-10 blazer, 150k. Running like crap after it warms up and engine under load about 30mph and up. So I wasted $70 on an O2, no change. I'd consider other things first if it has not set a code.
You can check for codes by grounding the ADL in the upper two ports with a wire, and the SES light will flash a code 3 times if set, and it flashes a 12 code first like the TRs (meaning good ECM). Of course they don't always code I'm sure. You can get a cheapo repair manual with the codes. I hope you can at least get the codes on the 95s also, I know some of the newer cars you can't.
I've narrowed my problems down to this, in order I will begin the repair work: Coil pack, distributer, CPI injector. I've already installed new plugs, wires, distributer cap, and set the timing (it was low). I'd replace these things first if you haven't already. If you adjust the timing while it's running, ground the ADL like you're checking the codes, so the ECM won't compensate. So there's my shadetree mechanic analysis