MOSFET Voltage Spikes
You are literally shortening the gate and source of your PMOS via a resistor, meaning that if you had only a DC signal as the supply voltage the MOSFET would be always OFF (open). The small signal that you see across R1, arises from the fact the MOSFET gate capacitance takes some time to charge and discharge, therefore for a very short moment, the voltage across gate-source is no longer zero, thus the MOSFET is shortly turned on.

I don't really see the purpose of your circuit. If you wanted to drive the P-FET as a switch, you would have to rearrange the AC source as follows (not sure whether that is what you want):

This circuit basically creates a voltage reference (12V) and switches the output voltage based on the AC Source.





The small voltage spikes can be removed via small capacitors.