The more problems there are at once, the more likely it is to be something simple.
Common issues:
Low battery power. It will happily show 12.5v or above, but when starting will struggle, dropping reserve down, where control units get upset, and show random faults. Often identified by the clock resetting to zero at each startup.
Main earth cable, from battery to body and engine. This is aluminium, and corrodes internally. Connections at each end can look good, but cable has high resistance internally. By-pass with jump leads to diagnose.
Water ingress to body computer connector, or into computer itself. The plenum chanber below the windscreen has drains (duckbills) that clog with dead leaves. The chamber fills with rainwater, which then overflows into the car, often onto the computer.
