A power supply, properly constructed, must provide DC
voltage unaltered even when AC mains goes so low that
incandescent bulbs dim to less than 40%. This even stated in
Intel ATX power supply specs.

Furthermore, it makes no difference whether the UPS is
'online' type or other type. Again, even Intel specs say
why. And that is the point. Too many will post user myths
rather than first learn how the equipment works.

What could have caused the OP's original problem?
Information provides is woefully too inadequate for any
responsible reply. Without an exact quote of computer
messages, then no one can answer the original question without
doing wild speculation. Even worse is the UPS reply which is
totally unjustified speculation; not based in technology
knowledge.

Even the MBM5 recommendation makes claims that are
technically false. Again, AC mains voltage must drop as
defined above and still MBM5 numbers would not change. This
assumes the power supply was properly constructed; a problem
with many clone computers. Spikes would never be recorded.
All this obvious when one has minimal hardware and electrical
knowledge.

Mercury wrote:
Could it have been the power?
You could instal MBM5 and use its PSU voltage logging ability to see
if you are getting spikes / brownouts.
UPS are dead checp these days - perhaps consider getting a true -
online UPS. The real cheap ups will not protect you against mains
power glitches, a true online one will since they run off battery
all the time.

So, it could be the mains power or the PSU in your system - this is
assuming the screen (?) flicker was caused by power and not the
system.