2005 Corolla misfires

Hi - 2005 Corolla; 38K, 4 cylinder. I had a misfire on the highway last week and the CEL came on. Took it to the dealers, they run the diags and determine it was a faulty fuel injector, which they replace. Got it back and it happened again the next day. I brought it in again and this time they suggest that I have a fouled tank of gas and make no repairs. Given that hypothesis, I'm driving through this tank but still getting misfires every 30 miles or so; CEL comes on after a misfire and then turns off. So far I've misfired on the highway doing about 60, getting on the highway, and taking a left from a stopped position. Any ideas what could be causing this behavior if the diags say nothing?