2000 Ford F-150 Rough Idle and performance

Engine Performance problem
2000 Ford F150 V8 Four Wheel Drive Automatic 108K miles

After sitting idle for about 8 months, with the same untreated 3/8 tank or so of gas, I drove my previously flawless p/u about 60miles at highway speeds with no hint of a problem. It sat overnight and today drove it about 4 miles with no problem when suddenly, while stopped at a light, the engine starting "missing." The tach would drop 500-700 rpms and the whole truck would shake. I thought it was going to stall for a minute. I pulled over and let it idle, with no change. Worsens in reverse, and inconsistent airflow out the exhaust. I know odds are good that either the gas went bad or I have water in the tank, (or both). Is there anyway to tell which? And how do I resolve?