Viewsonic CRT Brightness Problems

I have a Viewsonic P95f+ monitor.

19" CRT

I keep the brightness at medium level, but it regularly goes darker (Grey/washed out) and it gets harder to read the print (Seems blurry). I haven't nailed down a pattern, but I'd say every 15-20 minutes or so. It lasts 1-10 minutes, then goes back to my regular brightness.

The monitor is probably 5-7 years old, but I've had no problems until now. I recently put in a new power supply and graphics card in my PC, but I can't say if this started before or after those changes.

Could this be a problem with a driver/the graphics card/power supply, or is it just time to get a new monitor? Any tips on how to diagnose?

Thanks,
Eddie