Loses RPM and small poping sounds.

When accelerating it pops under driver seat and in the middle of the van. Usually between 15-30mph. It doesn't happen frequently until I drive and park restart a couple of times. It will basically drive fine for 30 mins at various speeds. It doesn't over heat but the coolant is a light brown. I have replaced fuel pump 2 years ago. Radiator 1 year ago. Recently replaced the EVAP emission lines and countless other things with the vacuum lines. Mechanic says it sounds like a blown head gasket but compression test checked ok. I usually replace the cap and rotor then all is fine but every 6 months or so they are corroded.