Upgrading the BIOS would be last on my last if it were my machine.

Let us know if the hard reset helps.

it remembers what was last selected.

Have you tried a different mouse pad?
Have you tried a different mouse?

I have seen this, but in my case it was a dirty mouse pad causing the optical mouse to always see an offset - even though the mouse itself hasn't moved.

A similar problem occurs if the optical sensor in the mouse gets a bit off, again causing it to detect a movement even though the mouse doesn't move.

In my case I just had to clean the mouse pad...

Another place (with older systems) is that oddball things can happen with a PS/2 mouse when the connector gets dirty.

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Cleaning mousepad hardly has any effect if the mouse is unplugged - as is the case with our OP.

Since OP is having the type of performance issue mentioned it seems likely that the
touchpad is on it's way out.

If a hard reset doesn't fix the issue a higher version of the kernel or re-compiling the kernel may or may not work.

-::-There should be a way to disable the mousepad in the BIOS.-::-
I'd be surprised if there isn't an option to do that.

i take it you're all talking about the touchpad?

Yes, thank you.

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Really? Actually, I haven't used Windows since last week but even today, Windows got selected, which is the last entry.

Hard reset seems to work for me as I haven't noticed this issue since 2-3 days(except in Grub menu). If the issue doesn't reappear for few more days, I will mark this thread as "Solved".

Thanks again for helping me out.

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in grub you can configure the default menu, try to set it manually, or reorder menuitems (if you wish). And yes, sometimes it can remember the last boot and set that one as default. But I think this is another issue, not related to this thread.

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Maybe, you are right but as it started happening simultaneously I believed that there is some link between these two events.

Unfortunately issue is still not resolved. Recently when issue occurred, I unplugged the mouse and disabled the touchpad but page automatically got moved to its bottom. I tried to move it up using arrow keys but page again got dragged to its bottom.

So you mean your touchpad is disabled, mouse is unplugged and page still got moved.
Probably I would try xev to check what's happening, although I'm not really sure if that will really help. Just a tip.
And another question: does it have a joystick built into the keyboard?

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maybe the Page Down key is stuck/broken?

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Since it seems to not be the touchpad, I suspect the keyboard.

I know it's more annoying, but you could try disabling the keyboard with xinput, and use an external USB keyboard to see if that resolves the issue next time it crops up.

If that's the problem, then you may want to replace the keyboard. Not very fun, with most laptops. If you do so, be very careful. It's easy to accidentally tear/slice a ribbon cable in two.

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Thanks guys for your help,

Nope.

I also noticed one more thing that while watching Youtube videos, sometimes it finishes the current video(Though video has not been played completely) and it starts to load the next video.

I am going to order a new USB Keyboard and I will check with it once I receive the same.