Oh yeah man I have been there!
I have an A15 with a 240 degree cam competition valve springs, datrats dizzy, big ports, extractors and 2 1/2" exhaust and a weber 38/38 DGAS downdraught carburettor. I had a problem where it would hit about 5500rpm and start coughing and spluttering pretty badly and wouldn't rev any more, after a couple of weeks or testing new parts, trying different jets, spark plug gaps and timing settings it all came down to two things.
1) I fitted a new fuel filter and when I disconnected the filter-to-pump line I noticed quite alot of air bubbles escaping from the filter outlet before any fuel came pouring out. I think I had air cavitating in my fuel pump causing incosistent flow into the carburettor bowl.
2) I put a new set of NGKBP5ES plugs in and left them gapped at .033" which is the stock gap, previously i had tried running .044" gap because thats what the datrats dizzys are capable of