Nope, this is not a transmission related issue. ATs have a dual point system that retards the spark in cold temperature. However the sensor is in the passenger compartment and only affects the timing at temperatures below 40 degrees. I don't think you northerners are seeing temperatures that cold this early in the season.
For AT's the idle is set while warm and in gear. Rpms raise when moving from drive to neutral or park as the engine unloads. A 250 rpm rise in that case would be normal.
Start the cold car and after a minute of idling note the idle RPM in park with the choke off. Drive the car to let the engine achieve operating normal temperature. Park the car and note the idle RPM in park. If you are seeing an increase, the mixture is changing, the throttle setting is changing, or a leak has changed the vacuum advance.
I would suspect a vacuum leak. If the leak occurs when the engine is cold and seals as the engine warms, I would expect the cold idle RPM to be low. This is contrary to Kully's description. I could see this happening if there was a vacuum leak at one carb that caused it to go lean when cold.
If the leak develops as the engine warms, the mixture leans out. Since the idle is high when warm, it makes me wonder what conditions were when the last time the mixture and idle settings were done.
BTW I have found that these engines never idled well at the "650 rpm" in Drive as specified. This worked well for 510's but not our Z's.
I suggest the following:
1. Check the carb mount bolts for tightness as well as the balance tube fittings and intake manifold bolts.
2. Remove the vacuum advance line from the distributor and block it with a golf tee.
3. With a warm engine in neutral, set the idle speed to 900 RPM and set the timing to 10 degree BTDC. Idle speed will be affected as you adjust the timing, so you may have to repeat this step several times.
4. With a warm engine in neutral and correct timing, adjust the carb mixtures per the manual.
5. With the car chocked and the parking brake applied, set the idle speed to 700-750 rpm.
6. With the car back in neutral, re-install the vacuum advance line. If the idle RPM changes significantly, troubleshoot. Vacuum advance should only affect timing when letting your foot of the accelerator at high speed.
I suspect that if your problem remains after this, the symptoms will change to a lower RPM at cold idle in drive. This may require more choke usage til warmed up.