Engine losing power?

My S-10 used to accelerate quite quickly, but for the last few months, it's been struggling to reach 60 in less than 15 seconds. It sat all winter, but all the fluids have been changed. I also changed the fuel filter, PCV valve, air filter is clean, and it just had a tune-up about 5,000 miles ago. There is the correct amount of air in the tires. I took it to the shop and they hooked it up to the diagnostic and found nothing wrong. Besides the acceleration problem, there seems to be nothing else wrong. Except for horrible gas mileage. I used to get 26, now I'm down to 17. Yikes! Please help. Throw out any suggestions you think would work! Thanks!

The past couple of days I've read through everything here I could find and it gave me quite a bit of help but my blazer stills has a problem.

About 5 months ago I bought a 1994 S-10 Blazer with a 4.3L Vortec engine code W. It wouldn't start and after checking fuel pressure found out the fuel pump wasn't working. I went ahead and bought a new AC/Delco pump, pulsator, strainer, electrical connector w/pigtail, sending unit, dist. Cap, rotor, plugs and wires, 2 fuel filters, changed engine oil and filter replaced the exhaust from the cat convertor back (old muffler and exhaust was rusted badly). Dropped the fuel tank and cleaned it out and installed the pump and sending unit. Put the tank back in the vehicle, installed one of the new fuel filters in the fuel line and put all the ignition stuff I bought in it. Put a fuel pressure gauge on it and made sure everything was working. I had fuel to the pressure gauge at the schrader valve so it seemed I was ready to fire it up. I tried that and it wouldn't start. I had hot blue spark but no fuel was getting to the plugs. I read in my manual that any problems found with the CMFI unit the entire assembly should be replaced. Well I was short on cash so I pulled the upper plenum and took the unit out, sprayed carb cleaner in the poppet valves and rusty nasty smelling fuel came out. Used some air (low) and blew everything dry and reinstalled it. Put on a new plenum gasket and bolted everything back together. After everything was bolted down and electrical connectors were back together I got in turned the key and within a few cranks of the engine it fired up. I let it warm up while I cleaned up the shop and took it for a drive. Everything was working fine. Drove it everyday to work for just over 4,000 miles then one day on the way home on the highway going up a hill I noticed I was losing power. Never did die but I had to step lightly on the peddle. Anymore than that and the engine rpm's fell. I made it home and when the weekend came I took it back out to the shop and decided to do some testing. Didn't know about this forum but I had the repair manual. Heck I can fix this myself. Yeah right. I read in the manual that if the fuel pressure didn't approach 61lbs. On acceleration that the pressure regulator on the CMFI unit was bad and it should be replaced and this past weekend I replaced it. Well after reading through a ton of threads here I found out that its a good idea to replace the fuel line assembly (nut kit). I didn't do that and I really didn't look over the plenum to see if there was any wash. Well it's still doing the same thing it was doing. I put a fuel pressure gauge on the schrader valve and here is what I found out. With the key on/engine off and the fuel pump running I have 60lbs. As soon as the pump shuts off the pressure drops to 58lbs but within a couple seconds climbs back up to 60lbs. I turned the key off and let everything set for 10 minutes and the fuel pressure then was 58lbs. I started the engine and the pressure was between 51lbs. And 58lbs. (Needle was fluttering back and forth). I increased the throttle to 2,000 rpm's and the pressure gauge needle flutters between 49 and 60lbs. Around 2500 rpm's and the engine starts to cut out, bog down or whatever. It won't throttle up anymore than that. It starts good and idles good and I can drive it in town but I can't take it out on the highway. I put a vacumn gauge on it and at idle it has 19 lbs/inches? And if I increase the throttle it increases to around 21 or 22 lbs/inches. I didn't pinch the return fuel line to see what would happen to the fuel pressure but I think I performed everything else. I'm really leaning towards replacing the fuel line assembly (nut kit) like I should have done when I replaced the CMFI unit. All the fuel pressure readings stay the same with the engine cold or when it's at operating temp. What do you guys think? Where should I go from here? Sure would appreciate any help you can give me. Sorry for the long winded post but I wanted you to know what all I've done.
Also wondering what would make the fuel pressure gauge needle flutter? Don't believe it's supposed to do that.

Hi
Well the gauge is going to move a little bit but did you check to see if you may have a bad trottle position sensor. Try this first and hopefuly this will help.