Started happening today with a Natty (11.04) update from kernel 2.6.37-12.26 to 2.6.38-1.27.
This is a regression since going back to the previous kernel fixes the problem.
System mostly unusable due to very high spikes every few seconds.

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root@orchid:/var/log/ConsoleKit# lspci -nn
00:00.0 Host bridge [0600]: ATI Technologies Inc RD790 Northbridge only dual slot PCI-e_GFX and HT3 K8 part [1002:5956]
00:04.0 PCI bridge [0604]: ATI Technologies Inc RD790 PCI to PCI bridge (PCI express gpp port A) [1002:597a]
00:05.0 PCI bridge [0604]: ATI Technologies Inc RD790 PCI to PCI bridge (PCI express gpp port B) [1002:597b]
00:07.0 PCI bridge [0604]: ATI Technologies Inc RD790 PCI to PCI bridge (PCI express gpp port D) [1002:597d]
00:09.0 PCI bridge [0604]: ATI Technologies Inc RD790 PCI to PCI bridge (PCI express gpp port E) [1002:597e]
00:0a.0 PCI bridge [0604]: ATI Technologies Inc RD790 PCI to PCI bridge (PCI express gpp port F) [1002:597f]
00:0b.0 PCI bridge [0604]: ATI Technologies Inc RD790 PCI to PCI bridge (external gfx1 port A) [1002:5980]
00:11.0 SATA controller [0106]: ATI Technologies Inc SB700/SB800 SATA Controller [IDE mode] [1002:4390]
00:12.0 USB Controller [0c03]: ATI Technologies Inc SB700/SB800 USB OHCI0 Controller [1002:4397]
00:12.1 USB Controller [0c03]: ATI Technologies Inc SB700 USB OHCI1 Controller [1002:4398]
00:12.2 USB Controller [0c03]: ATI Technologies Inc SB700/SB800 USB EHCI Controller [1002:4396]
00:13.0 USB Controller [0c03]: ATI Technologies Inc SB700/SB800 USB OHCI0 Controller [1002:4397]
00:13.1 USB Controller [0c03]: ATI Technologies Inc SB700 USB OHCI1 Controller [1002:4398]
00:13.2 USB Controller [0c03]: ATI Technologies Inc SB700/SB800 USB EHCI Controller [1002:4396]
00:14.0 SMBus [0c05]: ATI Technologies Inc SBx00 SMBus Controller [1002:4385] (rev 3c)
00:14.1 IDE interface [0101]: ATI Technologies Inc SB700/SB800 IDE Controller [1002:439c]
00:14.2 Audio device [0403]: ATI Technologies Inc SBx00 Azalia (Intel HDA) [1002:4383]
00:14.3 ISA bridge [0601]: ATI Technologies Inc SB700/SB800 LPC host controller [1002:439d]
00:14.4 PCI bridge [0604]: ATI Technologies Inc SBx00 PCI to PCI Bridge [1002:4384]
00:14.5 USB Controller [0c03]: ATI Technologies Inc SB700/SB800 USB OHCI2 Controller [1002:4399]
00:18.0 Host bridge [0600]: Advanced Micro Devices [AMD] Family 10h Processor HyperTransport Configuration [1022:1200]
00:18.1 Host bridge [0600]: Advanced Micro Devices [AMD] Family 10h Processor Address Map [1022:1201]
00:18.2 Host bridge [0600]: Advanced Micro Devices [AMD] Family 10h Processor DRAM Controller [1022:1202]
00:18.3 Host bridge [0600]: Advanced Micro Devices [AMD] Family 10h Processor Miscellaneous Control [1022:1203]
00:18.4 Host bridge [0600]: Advanced Micro Devices [AMD] Family 10h Processor Link Control [1022:1204]
01:00.0 USB Controller [0c03]: NEC Corporation uPD720200 USB 3.0 Host Controller [1033:0194] (rev 03)
02:00.0 IDE interface [0101]: Marvell Technology Group Ltd. Device [1b4b:91a3] (rev 11)
03:00.0 Ethernet controller [0200]: Realtek Semiconductor Co., Ltd. RTL8111/8168B PCI Express Gigabit Ethernet controller [10ec:8168] (rev 03)
04:00.0 Ethernet controller [0200]: Realtek Semiconductor Co., Ltd. RTL8111/8168B PCI Express Gigabit Ethernet controller [10ec:8168] (rev 03)
05:00.0 SATA controller [0106]: JMicron Technology Corp. JMB362/JMB363 Serial ATA Controller [197b:2363] (rev 03)
05:00.1 IDE interface [0101]: JMicron Technology Corp. JMB362/JMB363 Serial ATA Controller [197b:2363] (rev 03)
06:00.0 VGA compatible controller [0300]: ATI Technologies Inc Juniper [Radeon HD 5700 Series] [1002:68b8]
06:00.1 Audio device [0403]: ATI Technologies Inc Juniper HDMI Audio [Radeon HD 5700 Series] [1002:aa58]
07:06.0 Ethernet controller [0200]: Atheros Communications Inc. Atheros AR5001X+ Wireless Network Adapter [168c:0013] (rev 01)
07:0e.0 FireWire (IEEE 1394) [0c00]: Texas Instruments TSB43AB23 IEEE-1394a-2000 Controller (PHY/Link) [104c:8024]