1997 Corolla CE 1.4L Probelm?

My car keep thuding/studdering when you apply gas to it after you drive it for awhile. Once it starts the studering, it also makes a pop sound under the car (not in the rear) and when it studders and i apply gas i can put my foot all the way down and it wont go then it would start studdering and go for a bit and then studders and loses speed. Originally there was a leak int he head gasket into one of the spark plugs but i used a Head Gasket Sealer to stop it which it did but the studering was starting when it was getting water in the spark plug. I flushed out the entire system several times and for hours and no change. At times it would just die with out warning but only when i was going slow never when i was going past 20. When i had it in neutral it idle all right but once i put it drive it would studder and die. The spark plugs are still good. Suggestions ideas?