Hi Johan,
Methanol is a little harsh, try IPA (IsoPropanol) on a Q-Tip, also check your water flow by removing the hose from the input side of the laser and directing it into the chiller tank, the CW5000's have occasional pump problems so the water flow may not be what it's supposed to. All the symptoms suggest heat build up as the amps are rising (and it's measured on the negative side of the HT so the tube is passing the current ok).
cheers
Dave