Check the output from the regulator and service the regulator.
I had all kinds of cut out issues with my TS250/1 when riding and then turning the indicators and the lights on. Almost imposiible to keep the engine running with both the lights and the indicators on. Worse at low revs when approaching roundabaouts. On my bike tickover was poor at standstill but it did tickover. In the end I replaced the electromechanical regulator with an electronic device and the improvement was dramatic. Tick over much much better. Does not stall at junctions and roundabouts. Does not stall with the indicators one. Does not stall with lights on. I suspect it was never making a consistent voltage at low revs under load.