mmmmmRevs

Hi all, I have a 1989 RX-7 GXL, with some issues. I was having a mechanic work on it before now, but I am poor and have some time on my hands, so I am working on it. This is a BONE stock car, the only modifications that have been made are as follows:
-K&N drop in filter-previous owner
-Possibly a non-stock exhaust, as I had to add a cat to pass emissions
-My mechanic did something a little stupid... Instead of buying a used ACV (Air Control Valve... not 100% sure if this is the correct term, feeds air to cat and 6pi/vdi systems) and putting it on (the old one had failed) he did a janky fix by just running some hose to the exhaust from the air pump, so it would pass emissions. It saved me some moneyz, but I think it is causing problems now (besides non working 6pi/vdi).

Now, the problem: At warm idle (~2min after the temp gauge reaches fully warm) the idle drops to a very rough 500-600rpms, and lately has died with no input from me (in the past, it would stay on-barely) as I come up to a light, braking, clutch in/ in neutral. I have to position my feet in a heel-toe manor to apply gas to keep the car running. HOWEVER! once the car is FULLY WARM (20-30min drive) the idle runs higher and smoother, at 800-900rpms with no input from me.

I am thinking of purchasing a used ACV to gain use of the 6pi/vdi, and perhaps it will help with this? I know it controls idle somehow- it controls the 3k warmup thing.

Thank you for taking the time to look at this!