I just had that code about a week ago and took a day to track it down, turned out to be a leaky valve cover gasket... replaced and now there no code. Check for any leaks in you vacum system if you find one. then thats prolly whats throwing the code

99 SVT with resonatorless exhaust and open element intake
Sound System:
Clarion touch Screen Source Unit
Focal Components,Audiocontrol eq,
eclipse Subs and Amps