Could be fouled plugs from a tank of bad gas, but first check for diagnostic fault codes. If you have one for "cam and crank sync", suspect the timing belt has jumped two teeth. The computer will shut the engine down to protect the valves. You will often still get intermittent spark when that happens.
You might also take a little fuel and throw it on the ground, then toss a lit match on it. If the flame goes out, it's water. That happened to two cars in a row on the same day at the dealership I worked at.