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.