2001 Ford Windstar engine hesitation

Engine Performance problem
2001 Ford Windstar Front Wheel Drive 93650 miles

The engine of the Windstar will stall, pop, sometimes backfire. At first I thought it was water in the gas. Wife put something in tank to fix that. But as driving, it seems to get worse when going up hills or trying to maintain a steady speed. When going up a hill, you push on the gas, brief pause, then the vehicle lunges. Had a diagnotics done and the mechanic said it was misfiring. What could be causing this? With what I have found in my search is that it could be anything from a failing spark plug to the timing belt needing replaced. Any ideas? Can I fix this on my own?