Engine Performance on Cold Start Concern

440 with electronic ignition, Mopar chrome box. Dual 500 cfm Performer carbs, no choke (Not needed here in SoCal). I take it out for a drive every 2 to 3 weeks. It has always run just perfect, even on cold starts. About 3 months ago when I fired it up cold it ran rough and when I would give it some gas to raise the idle it would pop back in the muffler. After I put it in gear and started to drive it would run fine after 2 to 3 minutes. Module is in spec and I took the ignition box over to Autozone as they have the complete test kit from back in the day. They let me use it and the box tested fine. Same thing the next couple of times. Then it was fine on the cold start for the next couple of times after that. Then yesterday it happened again - ran rough and popped back in the exhaust when I raised the idle. Went for a drive and after 2-3 min all was fine. Thoughts?