My Blazer sputters when the RPM reaches 2500. It also sounds like I have a spark knock when idling. Runs fine under 2500rpm. When accelerating it also doesn't shift right.
Was wondering if I have a bad distributor or cam sensor or if a knock sensor would cause it to sputter. Does it have 2 knock sensors? Its an OBDII with the wires coming out of the side of the distributor cap.

I have no codes showing. Replaced the map sensor and Idle air control, fuel pump and changed the Nut and CPI assembly/. Recently tuned up, cap plugs, rotor, fuel filter, wires.
Someone told me it might be the fuel pressure regulator.
Running out of ideas.

Could be a lean-run condition brought on by a plugged fuel filter. Check and replace and see if the problem cures itself. :D In replacing your in-tank sock and pump, some grit could have gotten in that plugged the filter. There was a ton of pea sized stones and mud on top of my tank when I took it down. Hours were lost on cleaning it up before I removed the locking collar. Do you have a '95 with OBD-II ECU on the passenger-side wheel well? Chances are, you may still have a CPI based system that is subject to a pattern failure of fuel regulator, or leaking plastic fuel lines leading to the injector spider inside the upper plenum. If you have access to a scan tool, monitor your O2 sensor streams. Running really rich at idle when the engine pulls maximum throttle plate vacuum is almost a sure sign that the Regulator, Spider and Nut kit may need to be looked at next.

And no worries, this is a simple procedure to work through - you can inspect within 10 minutes by removing your IMTV under the "Vortec" cladding atop of the engine. The smell of raw fuel or clean spots observed with a flashlight through the IMTV hole is a sure sign your injector kit's gone on vacation. If you go in there, I recommend the expense and time to replace eveything and DO NOT reuse the upper plenum gasket. Buy it new, like everything else. Surgery should take you about an hour and a half. I did mine in about that time, but then took time to clean EVERYTHING I could see and touch and replaced my cap, rotor bug and wires while in there because access was easier. Altogether, it took me about 3 hours and I am a complete clutz. :D

he said he replaced the cpi and nut kit. cpi unit comes with a new regulator.
a bad O2 may cause a problem like that. my number 1 o2 was bad and once it hit 2,000, it started missin like crazy. like wolfox said, if u have access to a good scan tool, u can monitor ur o2's on the fly to see how the engine is performing.

I suggested it merely as protocol - to double check work an/or replace parts still under warranty if installed correctly and yet gone bad. But yeah, I certainly think monitoring real-time O2 data stream should narrow things down quick. If there are long-term fuel trims of 6% or more there is cause for concern, and if they register too rich/lean because of contamination or age, would warrant looking into next. How many miles have you got on those O2 sensors and catalyst? A plugged/shifted cat can also cause your trouble if it's restricting exhaust flow.

dont know if this helps but ..ive had a truck to do the same thing and it was the throttle position sensor (TPS) . ...but it turned up a code...sounds the same though