Given the time is takes to start failing, could it be related to some component overheating?
Enviado de meu Lenovo A6020l36 usando Tapatalk
I would open another one and see if that component that looks like it leaked goo all over the board looks the same or similar. I can't quite make it out but is most likely an electrolytic cap that went bad. A picture of a good board would help clarify things (or, you may find that you have other units that are on the verge of the same failure).
On the repair side, you can try and clean the goo off with a some isopropyl alcohol and a cotton swab (probably many swabs), Between that and figuring out what that component is, you might have a chance of fixing it. If not, those 700MHz systems are all illegal in North America, so there may be folks here who would happily sell you a replacement. I had a client just a couple of months ago who had one...said he was going to toss it but maybe he never got around to it.
