Check engine light flashing when engin stumbles

I noticed some stumbling of the engine when maintaining speed up hill or accelerating in a high gear. One day the check engine light started flashing and then stayed on. The day I was to take it into the shop to
get the codes checked checked, the light went off and stayed off for about a week but the engine still stumbled under the sames conditions. About two weeks later the light came back on and I got it into the shop to check the codes and it came back as an engine misfire. They sugested I change the plugs. As I had just changed the plugs 6000 miles earlier I thought this was a bit strange, but the service manager told me that if I had used Bosch platinum plugs that was most likely the root of the problem. So I changed the plugs again using the NGK plugs recommended in the owner's manual.
The engine purred likem a kitten
For about a hundred miles and it started all over again. I'm now thinking that it could be either the Crank Position Sensor, Throttle Position Sensor the Coil Pack. God help me if it's the control module.
Gas mileage is still fine and the problem with the stumbling is intermittent but chronic. It has yet to stall during one of these episodes.
Am I on the right track with suspecting of the sensors the sensors? Also I am aware that the timing belt is overdue for service. Can those things stretch to a degree that this kind of thing might happen? I remember having an old Dodge Colt with a stripped crank pulley and the engine would stumble in much the same manner.
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What year, model, engine? Did you change the plug wires when you changed plugs? You didn't say what the DTC was but if its P0300-P0306, I would suspect the wires first, then the coil.
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Misfire codes, especially those that can be felt as a lack of power and engine vibration, tend to be caused by the secondary ignition components (plugs, wires, coils).
Since the problem went away briefly and then came back, I'd suspect that you may have a carbon-tracked spark plug. When this occurs, a line of carbon runs down the inside of the wire boot along the spark plug from the tip to the hex, providing an alternative path for the electicity. In your case, there may have been such a line in the wire boot so that it only took a short period of time to recreate the path when the plugs were replaced. In that case, you'll probably need to replace the plugs and wires.
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You may need to replace the plugs and wires at the same time. What you describe sounds like the classic symptoms of a carbon-tracked spark plug and wire. When this occurs, a track of carbon runs down the porcelain of the plug from the external tip to the hex, providing an alternative path for the electricity. The carbon will be on the plug and the wire. If just the plugs are replaced, the track on the wire remains and will become active again in a short period of time. If just the wires are replaced, the track on the plug remains and will become active again in a short period of time.
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