NJ2011 Fusion

I am new to the Ford Fusion group in terms of posting but I have used it for answers in the past. I am driving a 2011 Ford Fusion SEL. The vehicle has 103,000 miles on it. About 3 months ago I encountered stop and go traffic due to an accident on the interstate. After coming past the accident and upon accelerating the car started to vibrate / shudder / shake - whatever verb you want to use - it wasn't normal. To the extent my cell phone vibrated out of it's air vent holder. I was a mile away from an exit so I pulled off at that exit and turned off the vehicle. It should be noted when the issue started there were no dashboard lights, there was no RPM abnormalities etc. After turning the car off and waiting about 10 minutes the car started and drove normally. The issue didn't repeat for weeks and the car ran smoothly as normal. It repeated after the same thing - stop and go traffic and upon accelerating. I took the vehicle to my wonderful mechanic that has been doing all the servicing on it. At 100,000 miles Ford recommends spark plugs so they changed the spark plugs, checked the brakes, the tire balance, cleaned out some carbon deposits, and test drove the vehicle - it drove fine. Again - a couple of weeks of normal driving and then it did it slightly one further time. This time back at the mechanic who won't charge me till he knows that the problem is solved and this time they worked on the throttle body which he said was pretty dirty and did a fuel injection treatment and cleaning. Yesterday driving between 0 - 59 mph the vehicle was fine. At 60 it started shaking so I slowed up a bit. Back to 60 and the shaking started. I accelerated a bit more and at 70 on the interstate the shaking lasted for about 3 miles and then went away. Driving back home - about 30 miles - the car was fine and after that short period of vibration and shaking it was fine. My mechanic is a bit stumped and so am I. There are again no warning lights, the RPM seems normal, the vehicle shifts gears smoothly. Just trying to figure out what is going on and if someone in the forum has experienced the same thing. Any help other fusion owners can give is greatly appreciated. Many thanks !