In a nutshell, system compromised somewhere allowing air in which is causing boilover...
Typical suspects:
- cracked coolant reservoir tank
- bad coolant cap (what are last 2 digits - hint .04 is latest revision)
- water pump
- reservoir overflow valve bad
Don't just start throwing $ at parts... start w/ a simple pressure test on system to identify compromised component. There are obviously others and some catestrophic but let's not even think along those lines.
Good luck
