Carbon gt battery or bms problem

Hey guys I have a carbon gt and I've upgraded the battery to a 10s5p LG hg2 pack and had it in for a few months but now I'm getting bad battery error... But the wierd this is c7 is dropping lower than all the other cells and c8 is staying at full capacity 4.2ish volts untill I stop and the board for a while and the cells balance... I've tried a few ways to test the cells and bms but too no avail. Any ideas on how to test wether it's my bms or battery?... When charged or left to sit after a ride the batterys will be balanced but just after a ride c7 will be much lower and c8 will be still at full voltage!!! Sounds like a BMS problem as one pack isn't being drawn from? Untill the bms balances them?

 

It will be the battery pack, it will have broken weld on some cells for C7