The flashing glow plug light is a "fix me RIGHT NOW" urgent check engine light.
There are many possible causes ranging from burnt-out brake lights to issues with the injection pump. You need to find someone with a VW-specific scanner to scan for codes, and then you'll know exactly what to do.
Since you're experiencing power issues it won't be the brake lights.
Could be pump, could be turbo. Code(s) will be an important clue.