1998 Cadillac Deville Hesitation and Backfiring

Recently I had a Cadillac garage install a new blower motor for the heater/air conditioner. They had to reposition the engine to make this repair. When moving the engine, is it possible that an electrical connection or a vacuum line might have come partially unhooked. My problem is this now: When I start thecar in the morning, it is a little rough but very smooth after a short time. As I drive away, the car accelerates smoothly and the engine sounds fine and normal. After I reach 40MPH or so and accelerate it, the engine gets very jerky and at times it will backfire. When I am driving 60 or 70 MPH on the road, steadily that is, the car runs very smoothly. If I accelerate the car even slightly the engine hesitates and gets jerky. My first thought was a dirty fuel filter but it would seem that the problem would be all the time. Then I thought about the evap canister. I have only looked for loose electrical fittings and whatever I thought they might have moved when working on the car. I have not looked under the car. Is there anything that could have come loose during that repair or do I have another totally different problem.