Newly rebuilt 351W problems

[Texasdoc]

I'm still having trouble getting my rebuilt 351W running smoothly.

Problem: The car starts instantly. Runs a little rough until it warms up. Then it runs well for about 15 minutes or so, then starts to get rough. For those 15 minutes, accelleration is good and smooth (haven't gotten on it much since I only have 50 miles on it total). After the 15 minutes, the acceleration is not as smooth. No backfires, just some hesitations through the rpm range. Also, the lifters *seem* louder and more clackety. If I let the car sit for an hour and cool off a bit, it seems to run well again for a few minutes, then starts to run rough again. Temperature guage reads a steady 190 degrees throughout all of this once warmed up.

What I have:
1982 351W (non roller) block 40 over.
Comp cams XE retro cam with hydraulic roller lifters.
Roller rockers set with 1/2 turn after zero lash on cam base curve - warm engine.
AFR 185 heads
Edelbrock AirGap Performer intake
Holley 670 carb with vacuum secondaries
100amp alternator
Proform distributor (HEI with built-in coil)
Mallory 6AL ignition box
Normal volume oil pump - warm idle pressure = 20, cruising pressure = 50
Castrol dino oil 10W-30.

The initial timing is set at 12 degrees with the vacuum advance disconnected and plugged. Intake manifold vacuum is around 14 at idle. Relatively stable with a mild fluctuation. It won't idle at less than 1k. Have sprayed everywhere with carb cleaner looking for air/vacuum leaks - none found. Starts instantly. I'm not sure what total timing is - not sure if I measure this with the vacuum advance attached or not. I'm also not sure if the 1200 rpm idle is causing some of the mechanical advance to come in at idle - not sure how to test this since I cannot get the idle below 1000 rpm. At 1000 it is rough, at 1200 it is smooth. I have adjusted the air/fuel to be 1/8 turn out from maximum rpm - same on both sides. This is about 7/8 turn out from seated. Secondaries are closed. I tried to go with the square slot, but it would then idle at 1200 with the idle screw backed all the way out - and the air-fuel screws were inoperative. At this setting, I would place my hand over the secondaries and the car would bog down.

So, I'm not sure what is going on. I'm not sure if this is a carb tuning issue, distributor problem, electrical connection fault, or mechanical engine problem. Why would it run well when it is warm, but rough when it is hot? Any thoughts? I am almost ready to change to an MSD distributor with external coil, buy a different carb (or fuel injection system). However, I would like to know what's wrong instead of just throwing parts at it.

Any help will be appreciated greatly!