273 stalls when down shifting

Interesting problem started happening to my 1965 Valiant 273 2 bbl, 904 auto, been converted to electronic ignition. Orange box is only a year old, and has been tested in other vehicles and appears to be working fine. Distributor is brand new. I rebuilt the carb a few months back. It's just a stock engine, daily driver. I hadn't driven it in a couple weeks as I was changing out the leaf springs, and pulled the distributor to install in the Cuda to get it running. After I moved the Cuda, I put the distributor back in the Valiant, adjusted the valves, and set the timing. It was running great, way better than before. Then it stalled and wouldn't start again. I checked a few things, turned out the ballast resistor blew. Changed it out, it fired right up. Then I was driving, had half a tank of gas, and it shut off as I came to a stop. Would hardly start again, but still started, but stalled right away. No fuel seen in the filter. Replaced fuel pump on the side of the road with a known good one. Started right up, went on my way, no more problems.
Well, this is where it gets weird. When it is idling in Park or Neutral, it idles perfectly. No sputters, choking, anything. When I put it into Drive, it drives like it should. I speed up through 1st, 2nd, and into 3rd, everything is normal. As soon as I brake to come to a stop, as it shifts down from 3rd to 2nd, it sputters, and again from 2nd to 1st, and then stalls. I turn the key, start it right up, and go on my way. It happens over and over.. why???
So, I checked under the hood for anything that looked like it needed attention. I noticed the positive side of the coil was a bit loose, so when I went to tighten it down, the threaded rod kept spinning, and fell out. Ok, I changed the coil with an extra I had, which I've tested to work. New coil installed, still has that same problem of stalling. It also now has a problem where it has no power under load, ie, climbing a hill, it just bogs down.

Whats happening? Point me in a direction.