1998 Ford Contour, 67K mi, V6-153 2.5L VIN G (24 Valve) SFI hiccups at 72mph

Here's the history:
Check Engine light checked by local franchise shop who advised to replace DPFE and EGR sensors.
Drove across town and back, engine overheated at end of trip, possibly had
been hot for MOST of trip across town.
Replaced thermostat - no difference
Checked for blown head gaskets, test positive, repaired heads, replaced gaskets, changed plugs, oil, antifreeze
Left shop, engine overheated within 3 miles, returned to shop
Shop discovered defective water pump and replaced, also replaced temp gauge temperature sensor
Drove down highway, car "hiccupped" at 72 mph, felt like engine died and then started or like tranny shifted down then back up or like a sudden hard jerk, then car drives smoothly at higher speeds. Slowed and sped up several times, car jerked every time I hit 72mph. Pulled off to check for whatever. Started back up and O/D light starts blinking constantly until I stopped and restarted the car. Light went out but car still jerked at 72mph. Noticed a hard shift from first to second and second to first. Check engine light on constantly.
New mechanic tests car, changes fuel filter and advises to change #1 O2 sensor before further testing.
Changed O2 sensor, no difference, check engine light on constantly.
New mechanic suggests tranny fluid flush, did it, no change EXCEPT check engine light is now off.
Car (engine/tranny?) continues to hiccup at 72mph.
Ideas?
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You didn't say how many miles. Check your emissions warranty. This may be covered.
Jeff
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67000 miles, I'll check that warranty.
Thanks

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says...

First, check out the forums through www.contour.org. They know the car.
Second, the blinking OD light means a trans problem was detected. It could be due to confusion from an engine problem, though.
Have you tried locking out overdrive or shifting to '2' or '1', and seeing if the hiccup occurs at the same constant RPM's, but at a lower speed? This might help narrow down if it's an engine or transmission problem.
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I checked www.contour.org, no luck so far.
The problem occurs whether the O/D is engaged or locked out. The stumble only occurs bewtween 72-74mph (always) and sometimes at 5-10mph.
Thanks

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Yes,
It does it when I accelerate slowly for the most part, I haven't tried a quick ramp up to that speed.

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