Front transmission seal. Cheap part, a little bit of a job to install.(Visualize pulling the transmision). The shut off could be either a leaky vauum line, or loose vacuum line, or the shut off on the rear of the IP is bad.
Take a vacuum pump and pull a vacuum on the shut off. If the engine just slows down, but does not shut off, you will need to replace that part. Ir it shuts down easily then there is a vacuum problem, of which there are many possibles.
I say a nice charge should do it, correctly placed it could end the suffering quickly.