Bogging and knocking

I have a 1998 Chevy S10 Blazer 4.3 Vortec V-6 4WD mileage is 124,000. It has recently had a full tune-up and transmission service. I have replaced the following: plugs, wires, cap, rotor, pcv valve, fuel filter, air filter. I have kept up on my oil cahnges as well. It has lost a lot of power and stuggles, knocks and pings. It idles ok but it has trouble getting up to speed and can barely mantain 65 mph on the highway. It bogs and is being held back. My gas mileage has gone way down. I checked to make sure my cat isn't plugged and it's ok. It obviously seems to be running lean. This problem came on very suddenly. I've had it scanned and all it says is random mis-fire. No specific codes come up. It doesn't seem to mis-fire at idle only under acceleration. I thought it may be the fuel pump as it is the original pump. But I was told that electric pumps don't slowly go. They either work or they don't. I am pulling my hair out over this problem and any help you could give my would be greatly appreciated.