Any diagnostic codes? There are a LOT of possibilities and if there are any diagnostic codes stored it may help a lot. Providing more complete information on your part will help too.
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Does this happen when the engine is completely cold or when it's hot or it doesn't matter what temp; does the car run find the rest of the time; is it smoking, does the exhaust smell of excessive fuel; when did you last replace the plug wires, spark plugs and distributor/rotor; is this a 7M-GTE or 7M-GE or something else?