chev. malibu 2000

i am an owner of a 2000 chev malibu that cuts out to a crawl after driving for a period of 30 to 45 min highway. the vehicle comes to a crawl as it loses power. then power returns after starting in neutral.
rides fine on streets. service department at maroone pines here in florida had vehicle for a week to try and determine problem and was unsuccessful in there determination of problem. they said it might be a thermostat problem. it was replaced by them and issue was not resolved. any suggestions or ideas of what it might be please respond .thank you , c.j
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Fuel pressure tested?, is the trans staying in a lower gear,? check engine lite on?, di?d it do it for them?, how could a thermostat cause this?, not enough info for any real help here, too vague.

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I agree with Shep. With info.so vague, I'd think a new battery would be as logical a guess as a new thermostat! Look seriously into Shep's hint re: fuel pressure--would be nice to leave a f-p gauge attached to check for correct pressure when first starting and then look at it quickly when it starts losing power WITHOUT shutting engine down. If pressure has dropped significantly at the malfunction time, look for faulty pressure-regulator or an intermittently clogging filter, as trash moves to block flow & then falls away when shutting eng. off & re-starting. Most difficult/expensive to replace would be an intermittently-failing fuel pump, as it would likely require fuel-tank removal plus high cost of pump. Good luck, s
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