Try this:
http://www.electrosport.com/electros...t_finding.html
it's a bit long but quite comprehensive. Or Sid the Squidmeister always seems to know something, if he's along soon.
Your alternators sound okay, could be the RRs or some grotty wiring. Voltage doesn't usually drop as speed increases, unless the charging system basically isn't working - should be maintained about 13-15V.