which specific p4 3.0 GHz. Depending on the model you may or may not have speed step. if it's P4 631, then it could be speed step. If it is P4 531/530, then it will not have speed step.
Download and Install IPDT(Intel Processor Diagnostic Tool) and post back the results. IPDT result will clearly tell us if it is speed step or if it is thermal throttling or something else.
good luck
-satyam