Thats Intel speedstep (EIST) at work. Its a technology that automatically lowers cpu clock speeds when the cpu is not doing anything strenuos. That reduces the load on the cpu, lowers temperatures, keeps fan noise down and lowers your electricity bill. Moreover, Speedstep is able to change the processor speed almost from cycle-to-cycle, which means for the user it will happen so fast as to be instantaneous.
Why do you want the cpu to run at max all the time? Are you experiencing any lags? IMO, there is no advantage in trying to disable speedstep unless you're experiencing lags. The only way i know that you could disable speedstep would be through your bios. Do the following:
1) Check the seetings of your Power plan in Control panel->power options. Verify that in Advanced Plan settings->Processor state, both max and min processor state are set to 100%.
2) Go into your bios configuration and look for "EIST". It may be under Advanced options or elsewhere depending on your system. Disable it there.