I'm marking this one as SOLVED!
The original battery negative earth cable goes to chassis and then onto gearbox. My original cable was frayed between chassis and battery so I snapped it off leaving the original cable between chassis and replaced the chassis to battery portion with a cable from ebay.
It turns out the ebay cable wasn't sufficient when on full boost, and cut the engine power. When I replaced the earth terminal to chassis to gearbox cable with one from an Alfa 147 JTD and cleaned up all connections the issue went away.