Weird variations in battery level

Hi,
I've charged my phone this night to 100%. I've gone on the bus and played Defense of Lushington Springs (tower defense) for about 30 minutes.
It burned about 50% which is a lot but I've seen these kind of burns before on some games. Phone was warm but not hot.
Then i opened Trial X3 (bike game) and played a level or two. The screen started flickering and Note's screen turned black. I've taken
the battery out and restarted it. It showed 1%. I've turned it off and waited for a while, turned it on again and it was on 50% again.
Later it also started flickering and crashed, this time it went back to 9% but then to 1% again.

Now this kind of behaviour clearly isn't normal, I'd just like to know if this is likely to be caused by the ROM, TrialX (lushngton springs did burn trough the battery but it was at least stable) or hardware (battery?). Note that i've tried TrialX before, like 20 levels at a time and it worked.

Battery levels: http://bayimg.com/HAJMIAaeE

My setup:
Note (of course)
ROM: AngrY by RevengeDevTeam V3 - 4.1.2
KERNEL: Philz touch 3.95

If it's ROM related (or perhaps even if it's not) I'd appreciate some info on which ROM would burn less battery and still be functional.

Thanks in advance