Early upshift into 3rd gear lockup and overdrive

My 99 Windstar was recently repaired for an intake leak. Following the repair the van shifts into 3rd gear lockup at low speed and fails to drop a gear even if a hill is encountered. Revs can drop to 1000 rpm without a
shift. On the hi-way, the overdrive engaes at 40 mph (70 kph) and will not downshift easily. Blipping the brake pedal or stomping the throttle will disengage the lockup or overdrive. Dealer suggests that we're not using the throttle correctly, but they were able to reproduce the problem one-time. However, they have not been able to diagnose anything. Suggestions or comments welcome! Thanks.

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