What do i need to stop my computer from rebooting in electricity fluctuations
Not wishing to state the obvious, but really, the first thing to do is resolve the power fluctuations if at all possible.

Assuming that's out of your control, then a decent UPS should be able to handle this by itself. You shouldn't need external inverters or stabilisers. 

To be honest, at first glance, it sounds like your initial UPS may be faulty or simply not up to the job. Can you be more specific about the nature of the fluctuations and how long they last?