What could be causing sudden loss of power on highway?

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Seems to happen when accelerating uphill. If I pull off and stop, it will be okay again for a while. This is an intermittent problem that mechanic was not able to reproduce. He replaced the throttle position sensor and it ran fine for about 400 miles. Then near the end of a 200mi trip I had to accelerate uphill and it happened again, I pulled off and stopped for awhile, then finished the trip on surface streets. Mechanic says to bring it in when it is happening, but in the last two weeks of around town driving, its been fine.