Maybe see if someone replaced the speakers in the cabinet with ones that were a little above spec... or someone removed the magnetic shielding. Anything that has to do with something that is magnetic, close to the CRT (in your case a projection lens assembly) is going to have that strange rainbow effect.
If there are other things going on, like for example, if when the screen gets really bright the entire screen kinda shrinks down... thats a different story.
Let us know if there are any other symptoms... check around for magnets... and be careful! There's quite a bit of voltage in there, even when the set is off and unplugged from the mains (I'm sure you know, but its good to re-iterate).
There are quite a few skilled technicians on this board... and if you can give any of them a better idea whats going on, by posting pictures, or trying to see if you can get any other symptoms to come out, it will probably be beneficial in the end.
Good luck!
Matty-