Surprisingly this is not on Eligius. It is on mainly DB and slush wouldn't even work. BTCGuild works best now, but it doesn't give good stats there. The hardware errors are everywhere, but pools do seem to affect speeds.
But even with BTCGuild I can't use the newest version because there's too many hardware errors and speed goes max 75%.
Is it normal to get clumps of rejected shares?
I think BTCGuild uses pushpool, which explains why its also screwing up.
But as a side note, hardware error != rejected. Do you have a high number of hardware errors or a high number of rejects?