What's the specs of your cam and are you using an auto or a manual? I'm a running a .040 over 360 around 9.8 compression with a cam of 241int/247exh, 532int/473exh lift center of 108, into a 904 with a 3200 stall converter.
I've been noticing that from cold through warmup I can get the idle to be stable but once I've been out driving around and I'm sitting at a stoplight in drive or in any gear I'll notice my vacuum gauge is lower and the engine is running a little rougher, I haven't really troubleshot it yet but I'm wondering if it has something to do with heat build up.