rpms fall off at cruising speeds

88 s10 blazer 4.3ltr z -vin i have replaced wires distributor cap,rotor,egr valve, egr soliniod,ignition coil ,map sensor,temp sensor,checked for blockage on fuel return,replaced fuel filter, and checked for slack in distibitor,checked screen in fuel tank also

Have you checked fuel pressure?
Have you checked engine vacuum when it acts up?
Have you checked for an exhaust restriction?

new muffler about 6 months ago removed conferter,have checked all vacumlines,havnt checked fuel pressure thou not sure how

On the throttle body is a fitting that looks similar to a tire valve stem. Connect a fuel pressure gauge to that fitting and turn the ignition to the RUN position, but don't start the engine, pressure should be 9psi to 13psi

On the vacuum, have you put a vacuum gauge on the engine and checked it when the vehicle acts up?

ok thanks iwill try that when i get paid i buy the tools to do so ant other ideas what about the knock sensor ,esc, or idle air control valve

Seriously doubt knock sensor would cause the problem, especially without the SES light on. The Idle Air Controller only controls idle speed.

is there anything else while iam waiting i can check i dont have a port to check fuel pressure its tbi have to buy kit

Sounds like you've replaced just about everything else to no avail, better check fuel pressure ;) After you have the results of the fuel pressure test, we can move on to the next step.

had some stuff come up cant get fuel pressure gauges when it acts up i smell something like old gas its not exhausted but kinda smells like it but not as strong i did pull fueline it seems like alot to me what does the fuel regulator drop the pressure down how do i no if it is out

A fuel pressure and leakdown test will tell if there's a fuel delivery problem. Takes about 10 minutes and tells you immediately if there's a problem. Harbor Freight has an adequate and substantial tester for about $13.

ok done it 9.8-10pnds when cycling the key on and off cranked up it is 11.4 and driving down the road it was 10 -11 even while acting up need to get a block in return line i just leave pressure gauge in the inlet line and block the return correct the haynes manual said it automatically bleeds down when engine is turn of because i have a TBI thanks