Starting to sputter. Troublshooting needed

[jestephens]

:-[
This last Saturday, I rode about 150 miles to meet some friends for lunch. Just before starting the ride home, I bought 2.71 gal of gas. About 20 miles down the road I noticed an overall lack of power, and severe sputtering in acceleration. The entire ride was interstate, with speeds about 75 (on the speedo), but at times I had trouble holding speed.
I stopped after about 50 miles and got 2.3 gal of gas and a bottle of STP fuel stabilizer and cleaner.
Idle seems normal, but any acceration is week.

So far I've replaced spark plugs, and inspected the plug boots for cracks. Even if a boot was cracked, it appears to have additional insulation on the heads to prevent any spark leakage as I suspected to find.
-No backfiring or other odd noise

Before I go to a mechanic and pay diagnostic fees, can you all think of other things to check? :-/
2005 with 45,053 otherwise trouble-free miles

Thank you in advance!!!!