mobile

Hi Guys, first of all thank you in advance for any answer you could give! it will be much appreciated!
Now, before anything I know there should be hundreds of posts about this issue but I was unable to find one that could apply to my scenario. But if you know any post that could be what I'm searching please just point me in the right direction :good:
Now, here is the problem:
We bought this cheap chinese tablet with andrioid 4.2.2 for my mother and it seemed to work pretty slow and some apps where not working correctly, there where a lot of adds even when she was not on an application, they just popped out on the main menu screen, and, when trying to register Instagram it displayed a message stating that there where too many users registered in the device. I tried to root the tablet because found that it could be because some temp files but it showed that it was already rooted... So my guess is that whoever installed the OS, did it with a "hacked" version of it.
Up to this point I have tried to do a hard reset but the issue is still going because it is still the hacked version of the OS, and I havent been able to find a way to completely wipe out the OS and install a fresh new version of Jelly bean 4.2.2
Can it be done with the device conected to a PC? or can it be done with a flashed memory card?
I know finding the firmware for a chinese tablet will be a major pain in the ass but, is there a version that could work for different models? (this is a Mobix MBX7 Plus, found the website mobix.com.ve but there is just no helpful info whatsoever)
Anyway, I hope you guys can help. Sorry for the somewhat long post
Again, thank you!!