Tachometer (Transmission) problems on 2009 Ford Taurus

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
I have a 2009 Ford Taurus X Limited AWD. When travelling at approximately 70 to 75 Kilometers per hour with an engine rpm of 1500 to 2000 the tachometer will occasionally buzz momentarily and the tach needle will bump up to approximately 2000 rpm and then return to normal. I have had the vehicle to the Ford dealership 5 times to repair this problem. They have replaced the instrument cluster, a computer chip, a valve in the transmission and some transmission hydraulic lines and still no cure. Am I going to have a transmission failure????? There are approximately 30,000 kilometersw on the vehicle. Ford says there are no codes. It happens almost every time when maintaining a fairly constant speed of between 70 to 75 kilometers per hour.
(1) Answer
(2) Comments
If it's a "fairly" constant speed, and you have some throttle every now and then, you could be disengaging the torque converter somewhat. This can be normal to some extant, but could be a problem, if that is in fact the cause.
Yes it is at a fairly constant speed and possibly a little throttle.
Could be looking at transmission failure?
Probably not. The transmissions are pretty solid. It may be normal operation.

You either have a CVT or a 6 speed automatic. I'm betting on the 6 speed. These transmission are pretty solid units.

You said they replaced "a computer chip". Did they reprogram the computer? If they did, make sure they cleared the transmission's KAM (keep alive memory). That's a special procedure that must be performed stand alone to get rid of any faulty shift habits that the processor has learned. I just went through this with a similar transmission and the odd shifting I was experiencing was rectified after doing this. Just a thought.