GOP 250CE Professional Multitool malfunctioning

Activity
Sector experience since
not enabled
Sector
Electrical installation
Written on
Hi all - I've had this tool since 2015 - It's had very occasional use (it still looks new!). Recently while using it the tool started to operate in a 'jittering' manner - almost like the power being cut in and out. It then operated at a very slow speed, the variable speed wheel making no difference. It only achieves a higher speed when it is initially switched on and rapidly drops to that slow speed with no way of altering it.
As a result I have replaced both the speed controller (1.607.233.437) - and when that didn't achieve anything, the switch unit (2.609.125.028) with Bosch genuine parts.
It's still the same, so I'm struggling to know what to replace next to try and get this tool working.
Any thoughts?