You can upload some sample shots to get better advice, but in general, M8 has well known "characteristics" in its sensor. If you have strong light source at certain edges of the frame, you'll get green streak. If you underexpose and the final image is "pushed" you'll get all sorts of banding and color noise goodies. ISO 2500 with cap on, AE would give you all sorts of funny noises.
Again, others can chime in with more specific advices, but it sounds like a typical good old M8 (and M8.2) to me. Best thing you can do is to expose right (or over expose a bit) when shooting high ISO (1250-2500).
I'd personally use up to ISO 640 for regular situations. I'd use 1250 like shooting with negative film (i.e. over expose if you aren't sure correct exposure). I forget about ISO2500.