s10 losing power

I have a 1999 Chevy s10 zr2...4.3 6cyl. 4wd.
She just started acting up a little: thought it had a bit of water in the gas, and added some fuel injector cleaner.

At a stop sign it 'coughs' and loses power, then picks up if you hit the accelerator. It will do this down the highway also, particularly if it downshifts and requires power.

I shut it off...after a couple hours started it again and the check engine light came on....and it was still acting up after driving it around 20 miles with the cleaner in it.

Could this be the oxygen sensor gone bad? I don't have access to a technician/with computer to get codes until Monday...and don't want to damage the engine.

Thanks for your opinions!

Some parts places check for codes free.
Get code and post back code.
How is fuel pressure?
How many miles on tune up?