quad 4 engine jerks at 60mph in 5th gear

Hi,
My car starts well, runs well when very warmed up, but at colder drive it jerkes and hesitates in 5th gear at about 60mph, goes away at about 70mph.
Any sugestions?
Thanks Dan 90 Grand Am Se quad-4 5 spd
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Bad spark plug is a possibility. Also, I think the ignition coil housings on some of the older Quad 4 engines tended to carbon track and cause voltage leakage - the old design was a black plastic, the new is white.
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Robert Hancock Saskatoon, SK, Canada
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Thanks
I'll start with a good complete tune-up
Dan

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