Dear JV,
May be your stabilizer is not doing the job well or its not build to tolerate the volt. fluctuation in your area.
Remember, connecting a stabilizer can also trigger rebooting the pc.
Let me know when the pc really reboots, can you list some events?
Your friend misguided you by saying to go for off-line one. if you are using an offline one then the issue will get more worse. An off-line system is delivering power from the main and switches to battery only when the power fails or drops to a particular level. Thats why it reboots the pc. The online system will be much better since they always using the power from battery to give output. But energy efficiency is lower here. The other one is Line Interactive system, it is designed to utilise both the on-line and off-line systems features. The special circuitry in it will use line voltage and switches to battery when needed like offline but the black-out will not occurs here like off-line. Offline systems are lower priced, the Line interactive systems are higher priced and the Online are less energy efficient(due to double conversion always) and the price comes in between. I think most are using the lower prices Off-line version so is the pc restart problem. Some off-line systems have UPS mode switch but I doubt they are not doing the function at all instance, so may be the pc restart sometime and not always. The UPS move switch actually just changing the lower and higher switch over voltage limits, hence a sudden voltage drop will restart the pc even with UPS mode switch on.
If you have that much problem, then the best one suits you will be a Line Interactive UPS but it will costs more. All types are available with most manufacturers.
TFL.