Vintage electric car circuit problems
From what I can understand of your circuit, it appears that the 36v supply can't provide all the current needed by the motor/controller, the 12v circuit, and the 36v contactor coil. The result is that the 36v supply voltage drops, causing the contactor points to open, reducing the load on the 36v supply, causing the voltage to rise, causing the contactor points to close, which causes the 36v supply voltage to drop, causing the contactor points to open, etc. ect..  This is what causes the opening and closing of the contactor points, as you report.