96 Stratus dies after being driven for a while

Hi all,
I'm writing for my daughter who's 96 Stratus (6 cyl. 2.5L) has been off the road more than on lately.
Her car was runny bad a while back and my son did a tuneup for her and
it was "running awesome" , as she put it. Then about 3 weeks later, she broke down on the highway. Car just sputtered out and couldn't be restarted. Got towed and the next day started right up. This happened 2 more times on the past 2 weeks. It's now sitting at my son's house and he's scratching his head trying to figure it out. He's not really a mechanic but usually has pretty good luck finding a fixing problems with cars. He said he let it run for about 2 hours in his yard and it seemed fine but my daughter is afraid to drive it for fear she'll get stuck again. Seems like it goes okay around town but as soon as she gets on a highway and it's warmed up real good, it's dies.
Any ideas what could be going on with this car? She's a single Mom with very little money and this is causing more hardship.
One tow guy said it's the head gasket but my son says he really doesn't think so because there's no water in the oil and visa versa.
Thanks for any suggestions. Margie
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