TH350 shifting issues

[AirSpeed]

My 72 GMC 4X4 with TH350 wont shift out of first on occasion. It usually happens the first time I drive it each day. It stays in first gear unless I pull the lever into 1st, drive to about 20mph and put the lever in drive, when I do this it will then shift normally the rest of the day. Once in awhile when it acts up I can stop, put it in reverse and then take off again and it will shift normally.
Any ideas what could be causing this? It doesnt happen every day, sometimes it will go a week without a problem. Someone suggested that it could be from parking on an incline. My driveway is sloped a little and I back it in so the hood is downhill, I parked it uphill tonight to see if that makes a difference, any thoughts on that theory?
Would buying a new modulator be worth a try?
Thanks for any ideas! Aaron.

[raycow]

That type of behavior is sometimes called "morning sickness" and I have run into it on other transmissions as well. I have never found it to be more than a minor annoyance. Of course, you may find it more annoying than I did.

If this is the transmission's only problem, and it goes away on its own after driving for a couple of minutes, I wouldn't bother trying to fix it. Replacing the modulator would be a waste of money, IMO. On the other hand, if the transmission is overdue for routine maintenance, like a fluid and filter change, definitely take care of that.

Ray