I have a pair of Yamaha HS80M, and one of them have this problem. After about 5 minutes after power-on, no matter if playing music or not, the woofer makes a really big uncontrolled movement resulting in an awful bump/noise. I think the tweeter does the same, but it is not as violent as the woofer. After that, if you play any music, the monitor is dead. Then you can power cycle and the problem repeats after roughly 5 minutes.
I made a small video clip to demonstrate:
If you have the same symptoms I have, it is very easy to find out if you have the same actual problem. In my case the problem was a damaged voltage regulator. To verify this, all you need is a screwdriver to remove the woofer, tweeter and the rear panel. And also you need a multimeter, any cheap one will work, to make a couple of measurements on the PCBs.
To switch the faulty voltage regulator, which can be a little tricky to desolder, for a new one you might want to have a little soldering skills and some proper equipment. Maybe you can do it yourself if you feel up for it, maybe you have a friend who can do it or maybe you can let a professional do the actual switch for you. It might not be so expensive to let a professional do it if you already know the problem and you have done all the required disassembly beforehand.
Below I will try to guide you through it with some pictures.
See pictures below
Service manual at ElektroTanya:
I've been told there might be a chance to fix this without the hassle of buying new regulators and desoldering. You can try to resolder the three voltage regulator joints to get a better electrical connection, as this might have been damaged. If this works it really saves a lot of work. Thanks gonkius for the suggestion. Any one trying this, please let my know as it will help others as well.
Also, there are several stories of successful repairs at my youtube area. I'm really glad to hear it!!! Good work!