[exwestracer]

I think the key is heat. You said it acts fine when cold. right? Also, if you putter around and don't hammer it it's ok?

I gotta think that as the aluminum base plate warms up and expands you are getting an interference issue with the throttle blades. You may not feel it, but it doesn't take much to raise the rpm like that. If you can get it back to 900, it's not a vacuum leak, UNLESS you are way off on the tune and have the idle set screw cranked in far enough to uncover the port vacuum slits in the throats.

Try running a thick gasket, or better yet, and plastic spacer under the carb if you have hood clearance. If you want to check the carb, pull it off and look very carefully at the edges of the bores where the throttle shafts go through. If you see any evidence of the blades rubbing the bores, it's getting stuck.

Hope this helps.