Some of the solenoids used on Hypertherm plasma systems can be disassembled carefully. I would attempt to take it apart and carefully clean internal components with a safe solvent...maybe rubbing alcohol, and re assemble. Keep in mind that if this does not work, you will have to replace the solenoid valve! I suggest talking with the great guys at Hypertherm tech service.....they talk people through similar fixes many times a day, they may have some tricks that can help you out! Here is the contact info:
Technical Service
technical.service@hypertherm.com
800-643-9878 Tel - Toll Free in USA
and Canada
Jim Colt Hypertherm