As I was still thinking about the mixture, the engine tone made a sudden and abrupt change. It was a low-frequency shudder and thudding noise that resonated through the cabin. I took one look at my airspeed and noticed it was dropping, as was my altitude. Jon, being a great supportive right-seat pilot, offered to help read out the engine roughness checklist to lessen my workload. Fuel pump, on. Tanks, switched. Mags, left and right bank, both sounded just as rough. Prop speed, adjusted. It made no difference. All the engine instrument indications from oil pressure and cylinder temperature to fuel pressure all looked good. At this point we had lost a full 500 feet and we had a critical decision to make. Do we continue on to cross the rest of the mountain pass, which is rising up quickly, or do we turn around and put the airplane on the ground in the middle of the valley?