From your description this sounds like an electrical issue--something major
losing contact, like a coil wire or a wire to the ECU, and it only does so
when there's forward pressure on it as you decelerate. Could be the ignition
switch or any number of other connections.

I had a similar problem a few months ago and it turned out to the high
tension wire from the coil, which was arcing at the dist. cap and fell apart
in my hand. Worth checking even though it seems unlikely.