A dying battery in these cars will cause a ton of problems. Trust me I know. I was using twins GT1 and killed my battery. When I boosted the car, the amount of error codes, and weird shit happening was beyond belief. I actually mini shit myself, as I though I reprogrammed something on the car. After it charged up, and I cleared all the codes, no more problems.

I would start looking in this sequence
1. Get it scanned and get the codes.
2. New battery
3. Voltage regulator