1999 Chevy Blazer Mixture/missing?

The car starts up fine, without fail. But for the first minute of driving, whether cold or hot, the car runs extremely sluggish, like its not getting any gas, the engine revs, and even almost backfires, sputtering and missing?. It is actually moving, but seems like it doesn't want to go, and I have to keep pumping the accelerator for it to go anywhere, then after about a minute or so, it just kicks in and runs fine, like nothing ever happened. Until, of coarse, the next time I start the engine and try to drive, it does the same thing. This has been going on for about a month. The engine light does come on, on occasion. and the gas mileage is horrible. Oxygen sensor? Seems like the mixture for the first minute or so is just out of whack. Any advise would be much appreciated. Thanks a bunch.