Engine

Nearly always an electrical fault. Start by unplugging sensor plugs and plug them back in again. There is also a problem area in the engine bay low down near the air intake - a multi-plug prone to water ingress - unplug, clean and re-assemble.
If you still have problems, it can be any of the engine sensors, the ECU, injectors or Coilpack.
Quickest way is to plug it into diagnostic eqmpt, or check each sensor in turn against known good parts.