Note that Desktop Pentium 4 does not have speedstep technology so even if you choose power saver mode, the processor will still run at its normal frequency. I believe that speedstep was only available on Pentium M back then.
There are 2 possibilities that I can think of.
1. Throttling due to overheating (Most Probable cause). Clean the fans, heatsink and repaste CPU. You are probably using the original thermal paste. It might have lost its thermal conductivity a bit leading to throttling.
2. You underclocked/undervolted the CPU (least probable cause) in the BIOS. If #1 didn't work, try to reset the bios to defaults and then update the BIOS.