Engine Performance problem
1999 Ford Ranger 4 cyl Two Wheel Drive Manual 145k+ miles

I have a 1999 Ford Ranger that is running rough. It is a 4 cylinder, 8 plug, 2.5 liter engine. I'm having problems with it stalling out and idleling low. When I turn the wheel to one extreme or the other, it stalls out. This has something to do with my power steering I know, but sometimes when I'm just driving up to a red light or making any kind of stop at all, stop sign, turn, etc. my truck will stall out. My friend said it might have something to do with the oxygen valve that controls the balance of oxygen and fuel that are burned. I took the truck to a shop and they told me that my head gasket was leaking and that was causing my 4th cylinder to miss, but the oil isn't milky and it would "knock" if I was missing a cylinder. What do you guys think?