Multiple misfire....Help

I have a 97 Blzr 4.3, Great take off & passing power, No smoke, starting or idle problems. But when cruising on level ground at anything above 40 mph, it surges and bucks. The tach also shows a slight jumping. Part stores and the dealer show the same code (Multi-misfire) sorry, but I didnt see the number.
I started with the obvious, cheapest and working my way up with know success. New plugs, cap, rotor, wires, coil, cleaned the MAF, had the control module tested, new EGR and today added a cat. converter with muffler to the list. Trans was re-built 1-2 years ago and shifts good. I've been the only owner and yes I have high milage, 302K but it does run excellant other than at steady speed. This is about a 4 month old problem and not getting worse.
Anymore suggestions ?????

Welcome to AF. Your symtoms sound identical to mine. http://www.automotiveforums.com/vbulletin/showthread.php?t=607221

Maybe is a timing chain or cam/dist gears, shaft, bushings.

97 is a bad year on ign switches.
Make sure you have good fuel pressure at the time it acts up.
What is your fuel pressure reading?

Good AC /Delco plugs?
What is the camshaft retard reading?
Unplug MAF sensor and see it it does better.
Good AC-Delco plugs and dist cap?
You need a good engine capable scanner to read out all sensor data and make sure all is in specs.
And read out what cylinder is missing on and how many times each cylinder is missing.

by any chance did you use after market plug wires? If so take them off a replace them with new gm plug wires. I made the mistake of getting after market plug wires and create more problems.

i had the same exact problem with mine missing and i replaced all that stuff and then i took it to the dealer and they said it was the cap and rotor. I had replaced it with a cap and rotor from autozone but when i put the gm cap and rotor on... it worked so try that. always try and buy dealer parts not aftermarket

Remove the distributor cap or look at the one you replaced and make sure the rotor is not hitting the cap. Mine had 250k on it and the distributor bearings get pretty loose with high mileage. If the cap and rotor are not alinged just right they can hit.

And If Camshaft Retard Setting Is Off It Will Give Missfire Codes.
With missfire codes you need to check what cylinders and how many times they are missing.
Low fuel pressure and a bad MAF sensor will also give missfires.
MT