Sound like its your ignition relay sticking or the ignition switch itself. This is what I'd do, Remove the panel under your steering wheel to gain access to your relay panel. With your ignition off (but the fault is keeping it on) remove relay 100, put it back in, when replacing it listen/feel for the relay closing, if it clicks and your ignition comes on again it'll prob be a faulty ignition switch, if it doesn't click and the ingition comes back on, it's the relay sticking closed. Hopefully this helps you find the problem!