Chevrolet Corvette Stingray C7 Forum - Tachometer bouncing dealer can't seem to find the problem

Good Evening everyone, I have a 2016 2SS M6 which I purchased used with 14500 miles in March of this year. About a month after purchasing the car I started to notice my tachometer needle bouncing especially during decel. As time progressed the problem got worse, now it's to the point that even while idling at a stop light the tach will fluctuate all the way up to 7000+ rpm this is also true for the HUD. I have had it at the dealership on five different occasions, I usually take the car in during the week and pick it up on the weekends for my weekend visitation. They have changed the cluster, and no change. They removed the dash and had found a loose ground in that area, well it worked well for about three days and started bouncing again. During that process they somehow damaged my satellite radio antenna. They replaced the shark fin antenna on the rear roof of the car with no result, and the tach is still going nuts. I took the car back on Tuesday 08/20/2019. As I normally do I wanted to pick up the car for my weekend visitation but this time they said they had to remove the headliner to correct the problem with the sxm antenna. I've looked on this forum and have found several possible fixes that have worked for others, well when I bring them up to the service rep he basically says I'll tell them but they don't think thats the problem. What have been the most common fixes for the crazy tachometer problem that have seemed to work long term or permanently? Thank you in advance for all your replies.