Rough idle in closed loop

[Kudzupatch]

Here is a stumper.

93 suburban 350, TBI. When cold/closed loop the idle is terrible.. Rough, idle drops, then speeds up but almost never dies. Sounds a lot like a cam with a lot of overlap.

Once it goes into open loop it smooths out and runs fine. No issues at all, just in closed loop.

High mileage engine but everything is in very good shape, I have kept in maintained. Cold air intake but otherwise stock. Checked the obvious things. Just put new plugs in and the old ones looked good. Grounds look good. No vacuum leak. Coolant temp registers correctly (using a computer to read). IAC working. Fuel pressure is 12 lbs. Data logging nothing stands out.

I have searched and find this is not an uncommon issue but have yet to find an answer to why. Vacuum leak is more common issue. I used starting fluid around all the gaskets, pulled and checked the hoses.

It used to run great but has slowly started the rough idle over the last year. Much worse in cold weather but again, once it warms up and goes to open loop it is just fine.

Totally stumped at this point.