Had a new transmission installed by a Chevy dealer. Changes first and second ok. When driving between 50 and 70 miles per hour up and down hills, passing in the flat, and pacing with traffic. Like in Atlanta Ga.. The trans shifts in and out of overdrive and 4 running gear. It's like it can't decide which gear to be in. When you depress gas pedal it will kick in the four barrel, then briefly into fourth, then right back into overdive and stay there. Keep pressing gas, and it goes into passing gear, when it should be in fouth. I suspect TV cable. You ?