960 Surging Idle, Stalling When Warm

[Bill960]

I recently bought a 960 with roughly 135k miles, though it had an engine replacement a couple of years ago. Initially it just struggled to climb hills, in the sense the car wouldn't respond to the pedal. But lately the problem's been worse, and

-When I'm stopped, the RPMs surge between about 500-1.5k. This happens even when the car is in park.
-After 10-15 minutes driving, the car will stall almost every time you come to a stop, or even very low speed.
-The power also 'drops out' when I go over a significant bump.

My instinct is that the problem isn't with the transmission, but something electrical or fuel delivery related. Before this I only owned a Vespa, which was very simple mechanically, so I'm a bit lost despite some research. Any ideas on what I should be checking?

[rspi]

Do you have a CEL? What are the codes? Check real good for vacuum leaks and maybe the MAF is bad.