Sounds like a smiliar prob I had with my old S2. the alternator was fucked, and having my full beam on at night and stereo made it all die and I had literally no lights for a few miles in pitch black, fuck that is scary! it even dimmed my clocks right out! it was like the batt wernt getting enough chrge, but when i booted it, it all popped back into life, anyway changed the alternator and that sorted it.

You might wanna change your batt as well, could have drained that a goodun.