My mechanic found white residue on the Secondary air pipe driver side head. He says the pipe threads are loose in the head and thinks the cylinder head may be cracked.

No coolant in the oil, does not overheat at all. I took the vehicle in because it stalled. It runs great then every once in awhile bucks hard. We just replaced the XYZ switch.

Has anyone seen this issue before?

 

If there's no problem caused by the head, I would leave it alone.
Do you have a check engine light associated with the SAI or bucking problem? If there's no codes stored, it can be tricky to find a direction to start looking for the problem.
Bucking can be caused by a lot of things. I have seen dropouts in throttle position sensor feed or signal voltage, or ground do that. Also, shorting on the 5 volt reference circuit will cause all sorts of weird problems. When there's a short in the 5 volt circuit, it pulls the voltage down on all the sensors that use it. Loose battery power and ground cables can certainly cause problems like this as well.
I think the best thing to do is, connect a scan tool and watch the live data when the problem occurs. Things like this can be very tricky to find sometimes.