I seriously doubt that it is healthy for any computer to run for a longer time with 100% CPU usage.
Computers in the DOS and Win9x days would be at 100% CPU usage for basically as long as they were switched on; and a modern one should be absolutely fine running at 100% for an indefinite amount of time provided that things like the cooling system are in good working condition. The biggest difference between CPUs now and CPUs then is the difference in power consumption between idle/sleep and full power - e.g. a 66MHz 486 consumes <5W at full power (and not much difference when halted) vs. ~5W at idle and close to 100W for a recent Haswell i7.
If I have a DOS guest running for some time in VirtualBox, with full CPU usage, OS X eventually will pop up a nice half transparent window. It tells me that my computer needs now to be shut down and that I should push the power button long enough to do so. It's the Mac version of the BSOD.
That suggests insufficient cooling. If it's a system that's been in use for a while, it's probably time to clean out the heatsinks and fans. If a new system is overheating from 100% CPU usage, then I would consider it defective.
Yes, definitely. The thing to advise is: Clean the heatsinks and fans, and then run a performance test such as prime95 over night to see that the machine indeed can run at 100% usage for a prolonged time. Only then I'd trust a machine that was prone to crashing under load again for my work.
I'm only just starting to learn asm so I'm not entirely sure how it works, but the source is available at http://maribu.home.xs4all.nl/zeurkous/download/mirror/dosidl....