Problems with ata driver on current / Asus P5Q-E

Hi,
I've been running a 7.1-p1 (amd64) system and and seeing very high interrupt
rates on irq19 which is shared between several devices.  Over 60% of one
core is used responding to interrupts.  The machine also hangs frequently.
 irq19 is shared by atapci1, atapci2, fwohci0 and uhci4...
the consensus response to my last question was that "it's probably the ata
driver".

Today I tried building and booting a GENERIC current kernel and got another
piece of information.  The freebsd-current GENERIC kernel does not boot;
shortly after probing the drives it says "Cannot setup DMA" several times on
the console and hangs.

This machine is an Asus P5Q-E... I believe it has two ATA controllers:
a Marvell
88SE6121 and a Silicon Image SIL5723.  Is one of these chipsets generating
interrupts that the driver doesn't understand and/or properly dismiss?  Can
it be put into a legacy mode that works?  This machine has problems in 7.0,
7.1 and current... I'm running out of ideas.

Thx,
Scott