Help: 98 Grand Marquis Losing power at Highway speed

Hello,
My Brother's 98 Marquis has stared to experience a power loss when driving on the highway. The problem occurs every 90 minutes; so he is pulling over
and letting the car cool down. On one occassion he tried to put the 4-way flashers on when he could not keep the car going 55mph on the interstate and the 4-way flashers would not come on. He tried them again after the car had cooled a bit and they worked.
I think the battery in the car is original. The car had sat for 3 weeks in a garage before this problem occured.
Any ideas or similar experiences? All input is always greatly appreciated
Thanks
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here's some input... teach your brother to type and use newsgroups then let him borrow your computer and post ALL the information/symptoms.
there has to be more to it.
Those two problems are not usually associated... the flashers would typically be affected by too low voltage and the car wouldnt run well either.... but it would not be a continuing problem because after a few time the car just would refuse to start.
that said .....I had a bad cruise servo kill all engine power on the highway. It was also heat related. The answer to that was to not use cruise, until i replaced it.

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