Erratic shifting in Powerglide

I need help diagnosing an issue with my 1968 Chevelle Powerglide transmission. It was rebuilt for me five years ago by a reputable company which unfortunately is no longer in business.I finally was able to install it in my car in August so it has only a few miles on it.It shifts just fine for about a half dozen shifts or so over a distance of about three miles and then will no longer upshift from low to high after that.If I let it rest for more than an hour, it will behave as before with about six shifts in about three miles and then stay in low.I have double checked all linkage and find it properly adjusted.I have excellent vacuum.I have varied the fluid level from full to just above the add line and back again.The fluid level changes have made no difference.Can someone explain why it behaves in such a manner and offer a suggestion on how to correct it?Thanks.

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The vacuum modulator and governor control shift points. It may be a temperature related problem with the governor that causes it to bind when warmed up. I had this problem after rebuilding the transmission in my Ford. I realize it is a different make and model but the same principles apply. I'm sure the GM guys at this site will have more specific information for your car. Good luck.

Found this from another site I am on . http://www.jalopyjournal.com/forum/threads/power-glide-wont-shift-into-high-gear.655177/

Update - I finally gave up and removed the transmission and took it to a local transmission shop. He called me two days later and exclaimed that it was "like a brand new transmission inside" and wanted further details of how it behaved. He finally discovered that the bolts in the governor were merely finger tight which allowed fluid to leak out of it and even more so after it got hot and less viscous. He tightened everything correctly, I reinstalled it in the car and now it behaves correctly. Case closed.

WOW. I am sure very frustrating with a lot of work for a simple fix.