A weird problem I have never encountered before

I just recently aqquired a 1995 Chevy Blazer, 2 door, 4 wheel drive, 6 cylendar 4.3L Vortec engine. Idles perfectly, runs good except when driving at a crusing speed the RPM's drop 300-500 and then jump back up constantly. I have checked the fuel pressure, changed plugs and wires, changed out the throttle control sensor, and talked to several mechanics and they had no idea what was causing the studder in the engine. It only does this while driving. It goes away while accelerating and decelerating. Also does not trigger a check engine code. Any thoughts our ideas on what this might be would be greatly appreciated.

Just a thought. Is your Air Condition ON. If so, you mite try turning it OFF and see if that stops the drop in RPM while driving. I had that problem once with my 97 blazer. I had a bad cycle switch that cause my compressor to go OFF and ON while driving on the highway. Changed the cycle switch and it was a done deal.

Good idea on AC Gabe25.
If it is not that.
Start with a full fuel pressure check.
How is the tune up and secandary ign end?
Are you sure TCC lockup is not kicking on and off?
I would get it on a good transmission/engine capable scanner and watch transmission and TCC clutch operation.
Check for past/present and history codes with a scanner.

The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
Until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold or hot and fuel pressure testing guide lines.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressures.
Cold start should be 64-65 lbs of pressure.
The 95 or yours is 55/61 lbs pressure
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 55/61 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75 or more.
And if it has a fast leak down after shut off you have a leak in system.
Post-back fuel pressure readings.

Remember a fuel pump pressure regulator controls the pressure and maintains a steady pressure depending on engine load and vacuum.
It has to be capable of producing higher pressure than the regulated pressure.

The fuel pressure controls pressure and can leak off or bypass pressure

The air conditioner is not functioning in this vehicle. The compressor does not engage even when you turn the system on. I appreciate the idea though.