Gauges going crazy, some rough shifting on 07 Pontiac G6 GT

I have a 2007 Pontiac G6 GT convertible. I bought it last summer with about
116,000 miles on it. It now has 122,000. The other day, the Traction Control
symbol lit up, then the car/padlock symbol appeared, then all of the sudden
the gauges went wacky. The speedometer needle would swing wildly from
50 to 0 and my dashboard gauges started going crazy, oscillating all the way
down to the 0 or off position, then back to normal. A warning bell would ring
3 times then stop. The SERVICE ESC and SERVICE TRACTION warning
indicators would appear on the display screen. There would also be some
minor rough shifting at lower speeds. The condition would last for a minute or
two then everything would get back to normal for a while. All would be OK,
then the symptoms would re-appear. When the car is turned off then re-
started later, usually all is OK for a while, but sometimes the problem occurs
as soon as the ignition key is turned. My mechanic said it was likely a
transmission control module, but that wouldn't affect the dash lights. He also
replaced one wheel sensor and checked the battery cables, but the problem
still persists. I love the car but I don't even know if it is safe to drive. Any
ideas?