Interrupt storm on ahc0 when drm module is loaded

I've come some sort of conflict between the drm and ahc drivers:
whenever drm is loaded I get an interrupt storm on ahc0.  My system is:

7.0-CURRENT #0: Tue Sep 25 18:08:50 BST 2007
/usr/obj/usr/src/sys/GENERIC i386

I'm running Xorg 7.3

ahc0: <Adaptec 19160B Ultra160 SCSI adapter> port 0xdc00-0xdcff mem
0xd9200000-0xd9200fff irq 16 at device 9.0 on pci0
ahc0: [ITHREAD]
da0 at ahc0 bus 0 target 6 lun 0

drm0: <Matrox G200 (AGP)> on vgapci0
info: [drm] AGP at 0xd0000000 64MB
info: [drm] Initialized mga 3.2.2 20060319
info: [drm] Initialized card for AGP DMA
drm0: [ITHREAD]

vmstat -i:
interrupt	total		rate
irq1: atkbd0	4036		0
irq14: ata0	58		0
irq16: ahc0	15004943	2824
irq19: fxp0	40961		7
cpu0: timer	10624196	1999
cpu1: timer	10606936	1996
Total		36281130	6828

Over time the rate for ahc0 grows until I get the "Interrupt storm
detected: throttling irq16" messages. I can trigger the problem by
changing the default colour depth to 16 bits in my xorg.conf file: this
enables the drm module to load and immediately the interrupt rate starts
to climb.

--
Bruce