Yes, if two plugs have failed the rest are probably going to fail soon. I would replace the ones that you can. Always use coolant and distilled water or premixed coolant to prevent corrosion. Tap or well water can have a lot of minerals and they will eventually clog the system. Some oldtimers used to use rain water.
Also make sure the coolant recovery system is working to prevent air from entering.
Last edited by rikard; 10-16-2011 at 08:22 AM.