2006 Mazda 3 AT warning light and downshifting

My car is having a problem that the dealer cannot seem to fix. When driving my car at speeds of 60+ my car will automatically downshift to 3rd. It skips 4th and goes from 5th to 3rd. It doesnt happen all the time though. My check engine light is on and when it was checked it was a code of "U0101" which is "Lost communication with the TCM". I am at a loss as to what is wrong. Can someone give me some ideas on what the problem could be? Thank you!