1968 C2500 engine dying

[gmc396]

I have a '68 GMC 2500 with a 396 engine and granny 4 speed. The last few days have gotten chilly; down to the high 40s. Truck starts right up with one pump, and the choke sets. After I drive a few miles and engine is warming up, when I push in the clutch, the engine dies. I'm able to restart by letting the clutch out. Pulling into a parking space at the end of my trip (about 3-4 miles) it died again. When I came out an hour later, it started fine. This time on the way home, it did not die.

I rebuilt the carb last winter and performed a tune up. It hasn't died all summer, but just started as the temp has dropped.

Any suggestions would be appreciated.