Processor Problem

I ‘ve NT server 4.0 service pack 6, I’ve a little problem, all 15 seconds my Cpu use 100 % of his capacity ( in Task Manager), and at this moment, the computer will be very slow !
And after 15 second the Cpu go down to 2 % , and will be faster….Please Help me !
Ps : I’ve Macaffee on this computer …