Check your IMEI in Nokia website to see if it's already over warranty. If it's the case it's pretty well worn.

I wondered if you've wrong concept of flashing - all apps previously installed by user should have been wiped out after flashing, how come you still found some unwanted apps after flashing? How did you flash your device?

Provided that you flash your device correctly (flashing via USB in flashing mode blah blah), you're reminded to flash your eMMC as well as the ROM. That would erase the data, but you don't need them for a device you've just bought.

Last but not least, switching of 3G/2G does consume huge amount of power. Therefore, sometime it's more power-efficient to lock the band to either 3G or 2G in order to avoid auto-switching. Of course, it might not be feasible for some region but it's a power saving tip.