Cold mornings and missing cylinders

[shuttermutt]

Hey guys, let me get some opinions here. It seems that since the weather has turned cold, I've got a problem with missing cylinders. When I fire it up in the morning, at least two don't sound/feel like they're firing right. Gradually, things smooth out and she'll fire on all eight, but it's not a friendly way to start the day.

I've pulled and gapped all my plugs correctly for an HEI. My wires aren't crossed or otherwise damaged (put these on about 6 months ago). I just can't think of any good reason why this would be happening.

It gets stranger, too. Once the truck's up to full operating temperature, I get a shrill whistle coming from under the hood when I let up on the gas. As long as the engine's under a load, there's no whistle. If I let up, the whistle comes on strong and I can change the pitch of the whistle depending on the position of the throttle. All this tells me is that there's a vac leak. The whistle doesn't stay, though. After a bit, it goes away and everything is fine.

The whole scenario goes like this:

- Start it up after sitting for several hours in cold, cold weather and listen to 6 cylinders do the work of 8.
- After about 90 seconds, all cylinders are firing.
- Start driving after about 90 more seconds.
- Choke remains on for about a mile then the idle kicks down to normal.
- Drive for about 10 miles at highway speed and the whistle starts.
- Drive for 4-5 miles more at highway speed and the whistle goes away.
- Every now and again, the truck insists on idling at around 2000 rpm when out of gear and it takes an act of congress to kick it down to normal again.

This is a 350 with an Edelbrock Performer RPM intake and a Edelbrock 1406 carb with electric choke. There's a 2-inch phenolic spacer between the carb and the intake. Anybody have an idea what the heck is going on?