Problem with Bally/Midway WACKO

My WACKO machine is fully functional but has one problem. Once it's
been on for about 20 mins, the screen goes to a white grid and stays
that way. The only way to fix it is turn it off for a while. I am
guessing it's just getting too hot inside. Should I try putting a fan
in the cabinet somewhere to keep it cool? Has anyone encountered this
with Bally/Midway machines and knows what the problem actually is?

Thanks, Jason