Are you sure your speedstep is working properly? The 5160 uses a P4-M which when at full pelt will get toasty...but steps down to as low as 100mhz I think when it doesn't need it.
Also you might try a program called SpeedswitchXP
....it allows you to manually control the power profiles. I used it and just set the profile for battery optimized to keep it around 1.6ghz....then if I wanted more power for playing a game or something just set it back to normal dynamic switching.
Also make sure your heatsink isn't full of dust. I periodically blew a can of compressed air through to keep it clean.