I know a guy who owns a JV-2080 expanded with a Hip Hop board. When he plays sequences on it with any other instruments than drums,it sounds OK. But when he uses drums + other instruments, it happens exactly the same thing you describe.
If he tries the same sequence on a JV-1080 - it works fine.
I don't know what it is but after reading some mails on the problem there is the possibility that the JV-2080 is even slower than the JV-1080 on account of the amount of information it has to handle.
Could it be because of the expansion board!?
Do you have an exp.-board in it?