My '93 245 has a rough idle problem on cold days - starting at about 20 F, getting worse as temperature drops.

Startup and initial driving are fine, but after about 5 minutes of driving (usually at a stoplight on my drive to work), the idle surges about once per second. On very cold days it may actually kill the engine. After the car is warmed up fully, the problem seems to go away.
Also, if I place the car in Park and take my foot off the brake, the surging will dampen out and the car will idle normally.

I recently replaced plugs, wires, cap, rotor, airfilter, cleaned TB, and replaced flame arrestor. There has been no change in this surging problem. At least there are no check engine lights!

The heated air intake hose has been removed (rusted away), but the airbox port is still open.

One strange thing I discovered is a disconnected brace between the left motor mount and the intake manifold. This brace also has the IAC connected to it. A previous owner / shop must have reinstalled the intake manifold with the strut misplaced, then ignored the mistake. I think there's more engine movement because of this, and it also messes up the routing of the IAC and AMM ductwork. I'll fix it if/when I remove the intake manifold. Could this loose brace be related to the surging?

I'm thinking it's most likely related to the brake booster. Any ideas / explanations out there?

Thanks!