My 1986 944 Turbo had the same problem.  It was a vacuum hose coming out of
the intake manifold.  The fitting in the maniforld is glued in and it had
come loose.  This hose goes to the idle stabilizer.  I think it is located
just between cylinder 1 and 2 on the intake manifold, it is easy to see near
the top of the intake.  I cleaned the metal fitting and epoxied it back in
and it now runs fine.

Mike T

My 944S2 also has similar problem. When it's idling, the enginn rpm
would jump up and down and gradually fall down to between 800 and 600
rpm. That's when the engine would stall. The car is at the shop now.

Eric