92 S-10 Blazer engine revving/lobing

4.3L engine suddenly began revving while driving at highway speeds, as if pumping on the accelerator - it causes the tranny to continually down/up shift. At a stop the brake has to be held hard because the engine continues to rev up and down causing the truck to try take off when it revs. Attempted remedies: Check for vacuum leaks - none; check fuel, replace fuel filter - no improvement; check EGR operation, clean, reinstall - no improvement; check fuel pressure - 60 psi (normal is 57-62) - no improvement. Engine continues to rev like someone is pumping on the gas pedal. While it's occuring there is no movement of the throttle mechanism - so it must be occuring by fuel delivery at the injectors? No check engine light. Check engine light code reader indicates normal code (12). When I manually open the throttle to hold RPMs up, it continues to rev at the higher RPMs, but the frequency increases proportionately. What could this be?!

First thing, if the a/c or defrost is on, turn it off. It can affect idle. Next when it's running, disconnect the IAC connector. That takes the computer control of idle out of the picture. Bad TPS or MAP sensors can also cause this. If you disconnect any of the sensors, the SES light could come on and set a code.
You said fuel pressure is normal. Does it hold steady with the engine off? If not, could be leaky injector, regulator or lines.

The ac/defrost were not ever on during this. I disconnected/reconnected the IAC, TPS, and MAP connectors one at a time while manipulating the throttle to keep it running because it wanted to die when anything was disconnected. While holding the throttle in a position to keep it running, it still fluctuated in RPMs (revving) as it did before. Fuel pressure did hold steady at 60 psi. Bad thing now is that after doing all these tests and shutting off engine, it now will not restart - most likely the same root cause (that I am trying to figure out).

Having it not start may be the best thing. What ever was going bad , now is. You can start at the basics, fuel, spark, timing. It would also help to have a scanner on it also.

Thanks for the response/input. The problem is solved. It was the CPI unit. I removed, disassembled, thoroughly cleaned, reassembled and reinstalled it. There was a lot of carbon deposits inside of the plenum area, some must have gotten into the CPI unit, causing the problem. The carbon deposits probably occurred when the EGR failed about a year ago - the EGR problem was corrected at that time without further investigation. I was fortunate that the CPI is still operational - a new one costs about $300.00.