THE MOST IMPORTANT PART, check mo ang mga gasket nung manifolds and hoses leading pressure in and out of the engine for possible leak. Change the gasket para sigurado ka na talaga, anyway mura lang naman yan. i know someone who had the same problem like yours, parehong pareho talaga with all those symptoms, procedures and solutions he had resorted to, gumasto rin ng malaki, only to find out its the gasket of manifolds. UNBELIEVABLE?!!! YET ITS TRUE. Good Luck.