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ASSUMING, of course, you've changed the battery cables by now, and the engine ground strap:
I went round and round with this problem a while back with an old Belair 350. There's a few possibilities:
1. Choke out of adjustment
2. Cracked Heads
3. Faulty Head gaskets
4. Faulty vacuum hoses.
5.Warped valves which don't act up till the engine's hot.
The best thing you can do is give a cold and hot compression test to try to localize the problem to a specific cylinder bank.
It would also be wise to run a vacuum gauge till you discover the source of the problem.