1985 BMW 735 timing chain

Hello 85bimmer735

A stretched timing chain can affect both valve and ignition timing. If the wear is severe and the chain jumps timing, the engine may run rough and/or possibly backfire but why 15 minutes?
I'll suspect a bad catalytic converter.
Raise the car, using a soft hammer tape on it, listen for any rattling noise.
Or check intake vacuum at idle. If intake vacuum reads low and continues to drop, inspect the exhaust system.