replace your plugs and wires. its the misfire. if you plug wires are real old its probably arcing to your engine block and not firing in the cylinder... may i suggest you fix this quickley so you don't toast your entire emission contol system and have to replace that too. this happened all the time with my old VW (WORST PLUG WIRE DESIGN EVER!!!!!) and this sounds very familiar to me. when was the last time you had a tune up?