You may have a worn throttle shaft bore in the baseplate. After it begins, wiggle the throttle shaft to see if it changes pitch or is eliminated, maybe a couple of drops of 30 wt
oil on the shaft. Many times the return springs are incorrectly attached at the bottom of the linkage causing the shaft to bare against the rearmost surface of the bored hole in the base plate. The spring is best attached so it attaches and pulls to the front of the engine (conventionally mounted 4bbl). Mounted at the same point as the throttle cable. If this is a new carb it is probably something else. But it is heat realated as you described, it is caused by expansion or warpage of a carburetor component. Also squirt a little carb cleaner in the air bleeds while it is running.