93 F150 developed very rough idle, power loss, gas usage

Hello. I have a 93 F150 4.9 inline 6, auto with 200.000 miles and it ran great last fall but sat a lot over the winter and now has developed a very rough idle and power loss. I checked everywhere for vacuum leaks to no avail and I sprayed solvent all around the intake while it was running in search of an intake leak and found none. It does smooth out some with acceleration but still has a marked power loss and sorta misses. I have noticed it using more gas. I did just replace the cap, rotor and plugs but no help. Also ran a couple cans of stp thru it. Im looking for suggested solutions. I tried to get the auto parts stores to check codes on it but they all say its to old for there tool. The haynes manual says vacuum leak or bad plugs. Would appreciate diagnostic suggestions, thanks.

I"d say you've got some bad gas in it if it sat through the winter and wasn't run.

I disagree that it could be bad gas, unless you had almost no gas in it. A few gallons of gas in a tank are not going to go bad over 6 months.

I tend to agree that a vacuum leak is a likely culprit. There's a lot of plumbing on these engines. How did the plugs that you replaced look?

sounds like an over zealous egr to me, wont cost anything to rule it out. when its cold unhook the exhaust line to it and see what it does. have to plug off the port on the valve itself to drive it like that. duct tape will sufice. as long as the exhaust is out of the picture it shouldn't get too hot. Is the light on? I realize the ecm is not nearly as smart as they are now days, but if things get way out of wack it does set the light on.