2000 Regal SLE - Trans. constantly downshifting

[Matt M.]

2000 Regal LSE
Not super-charged
Transmission rebuilt @ 125,000
203,000 miles on the Odometer
Work Car, Daily Driver - 65 miles mostly highway/freeway.

I'm routinely on and off the gas in my commute, usually in busy traffic, but never heavy braking. When I'm on the gas, I'm gentle with the throttle usually easing into the pedal. I've noticed in the last 3k-5k miles that car has started to downshift immediately (not hard), RPMs rise, car doesn't picks up speed and then shortly shift into a higher gear. Even at highway speeds - where I would be running 60-65 mph, the car shifts down out of OD, I don't even pick up any additional speed before it shifts back into OD again. The transmission is doing this constantly even at lower speeds.

I don't recall it doing this in the past. Any thoughts or am I being overly cautious? At 60-65 I would think the engine would be able to pull itself without having to downshift.

Car has been throwing CO2 codes off and on - not sure if that is causing any thing related or not.

Thanks