Have a reputable transmission technician check the transmission linkage adjustment for proper detent positioning. (To make certain the steering column selector lever/shift lever is positively in “D” and the transmission linkage underneath on the side of the transmission is positively in “D.”)
I suspect your torque converter is the culprit. ( A one-way bearing like clutch.) Keep driving it and if it does get worse after time, see a transmission specialist.
Check your rear wheel seals for leakage and/or brake pad/lining contamination. This can be done with a flashlight by examining the wheels and tires from the back side of tires and wheels.
If you’re driving the vehicle and it is cold, the engine shouldn’t return to normal idle speed until it’s warm, so that is normal.
Check the vacuum line/hose from the intake manifold to the vacuum modulator (vacuum can) on the transmission. Look for breaks, splits, cuts, burnt spots from exhaust pipe/manifold that might cause a vacuum leak. The vacuum line should dip down below the level of the modulator so that any condensation will not enter the modulator. Remove vacuum line from modulator and check for moisture, gas in modulator. There shouldn’t be any.