I have a 99 F350 with a 7.3L Power Stroke turbo diesel. when I accelerate to a steady RPM I hear a hissing sound like air is being released. At the same time the engine seems to lose power and the rpm's drop. This happens anytime i hold a steady rpm weather driving or in park. If I accelerate it stops until the RPM's hold steady again. Also, on an immediate cold start it doesnt seem to do it. After 30 seconds or so, as the engine warms up the problem appears. Does anyone know whats wrong? Any feedback is greatly appreciated.