Replaced battery - now getting random shut downs :(

Hi all, I have a late 2011 15" Macbook Pro. I replaced the battery with one purchased from ifixit. The install was straightforward however I'm now getting intermittent random shut downs - the computer will lose all power (screen turns black etc). If I press the power indicator button on the side of the computer it will show no power for about 30 seconds. Then it will show the battery strength and the computer can be restarted. This will happen with any battery strength remaining, but usually around the 30 to 50% range. I have reset the SMC and the PRAM with no improvement. It's not happening very frequently - about once a day or so, but is becoming annoying. When I first installed the battery, I let it fully charge, and then ran it down almost to zero before recharging it? Any help is appreciated - not sure if this is a battery issue and it needs to be replaced?

Thanks!