I think my Conn Pan Am has the same keywork. I just took it to the shop 2 weeks ago for the same reason.
Check the screws on each end of the low Eb-C key rod. One has probably worked its way loose. My repairman says that is typical for old Conns as he plays a Pan Am himself. He said he put a drop of nail polish on the end of the screw after retightening mine. Good luck.
Split bell keyed horns, such as your NW Series II, seem to exhibit this problem more than same sided key designs. This is very much exacerbated by any slight leak in the horn and/or a mouthpiece that has a too small chamber for the horn. The cork trick definitely helps with "motor boating" problems.