Nissan Repair: n15 1.6 pulsar problem, vacuum hoses, rocker cover gasket

Question
I did a service on a 1997 n15 1.6 ltr pulsar, replace rocker cover gasket, and changed oil and filter, nothing else after 5 kms of driving I feels sluggish at times and when this happens it blows white smoke. prior to this the car was perfect had all services done and up to date either through nissan or independant repairer. It's not using any water /coolant no bubbles when take off radiator cap, no milky residue in oil and transmission oil is fine. so losing nothing in the short distance travelled since service on saturday 20th feb 2010. I read somewhere the pcv valve or vacuum hoses could be off or blocked, checked them the pcv valve was dirty so cleaned with carby cleaner and took car arounf block and no smoke and drove perfect, no the sluggishness is back and white smoke again. I used 15/40 grade oil during change, also rechecked pcv valve I it appears to be stuffed I can blow through both ends. could the pcv valve have been causing the problem?  as mentioned the car was perfect before the basic service.

Answer
Ronald,

I would use thinner oil.  It sounds like the oil is not running back to the pan and staying up at the head and this allows the valves to suck oil down the guides and the oil also contaminates the PCV and the intake tract.  The oil is causing the poor performance since it fouls the plugs.  Try using 5W30 and see what happens.