I am having exactly the same problem since the last official version of SL Client. It is also the same problem with the Beta version now out. Not a problem on my Mac PowerBook G4 or G5 tower computers but happens every time I am on SL even with the lowest visual settings.
Hardware Overview:
Model Name: Mac Pro
Model Identifier: MacPro1,1
Processor Name: Dual-Core Intel Xeon
Processor Speed: 3 GHz
Number Of Processors: 2
Total Number Of Cores: 4
L2 Cache (per processor): 4 MB
Memory: 8 GB
Bus Speed: 1.33 GHz
Boot ROM Version: MP11.005C.B08
SMC Version (system): 1.7f10
Serial Number (system): RM7282SRWS4
ATI Radeon X1900 XT:
Chipset Model: ATY,RadeonX1900
Type: Display
Bus: PCIe
Slot: Slot-1
PCIe Lane Width: x16
VRAM (Total): 512 MB
Vendor: ATI (0x1002)
Device ID: 0x7249
Revision ID: 0x0000
ROM Revision: 113-A52027-202
EFI Driver Version: 01.00.202
Displays:
Cinema HD:
Resolution: 2560 x 1600
Depth: 32-Bit Color
Core Image: Hardware Accelerated
Main Display: Yes
Mirror: Off
Online: Yes
Quartz Extreme: Supported
Rotation: Supported
hp L2335:
Resolution: 1920 x 1200 @ 60 Hz
Depth: 32-Bit Color
Core Image: Hardware Accelerated
Mirror: Off
Online: Yes
Quartz Extreme: Supported
Rotation: Supported