Bold 9900 keeps shutting off and wont power on

I’ve had a Bold 9900 for 21 months and recently its been screwing up.

One night it shutdown and would not turn on until I plugged it in. Since then if i am on the internet or one the phone when its not plugged in the phone randomly shuts off. If i try and turn it on it gets part way through the load up bar and shuts down again. It only turns on if I plug the phone into the socket or PC. It also suddenly shows and empty battery even though it previously had a full battery

Sometimes it also shuts off and the red light flashes the same signal over and over (a single flash, then three then a single one).

I have tried wiping the phone, downgrading the OS but none of it has worked. Has anyone seen this problem before?