This is going to sound crazy, but go with me.
Go Into your kitchen and grab some aluminum foil, wrap the carb in foil not to block the top of course.
Drive and see if the problem is gone. If it is, check your heating source (not sure what exhaust manifold you have, if stock, check if the damper moves freely.)
You may have a carb icing issue.
Your conditions sound identical to our Olde Charlie.
