Originally Posted by mellowyellow01vert
Sounds like a fuel problem. Have you changed where you get your gas over the past few weeks?
Had the same exact issue when I filled up with a bad batch of premium from Mobil in my L98 powered C4. Went through the whole "tune up" process, new plugs (only 15K on the old ones), checked wires cap and rotor - all good, checked for EGR function (checked fine), verified ignition timing - right on, ran a batch of fuel injector cleaner with pressurized can directly into the fuel rail - no change, (new injectors had been replaced less than 20K miles ago), no codes thrown, check function on the Diacom - all appeared correct, replaced O2 sensor, no change in 30-40 mph operation, except mileage did increase by about 2 mpg, and replaced the MAF sensor with a known good unit - again no change.
Ran tank almost dry, and filled with Shell Premium, and within a mile the engine was running like it should.
I have a theory (and it is only that at this point) that with the usage of 10% ethanol, some stations are salting in water as a "filler" and pocketing some extra change. Ethanol absorbs water (and then mixes with gas), and would be easy to add, and most people would not notice the difference. Back in Illinois, when Ethanol first came out we found several stations increasing the percentage of ethanol significantly. The law said no more than 10%, but we found some stations always ran 16%-18%, and others in between. The Shell and Amoco (BP now) always ran on the money at 10%.
In other words, try a different gas.