High Side Switch Flicker during Power Up
Those R values seem rather high for gate control... especially if you plan on using it for PWM.  I'd drop those resistor values by at least a factor of 10. Lower resistances will also mean the MOSFET will switch faster and run cooler when you PWM.

Incidentally, R1 is also unnecessary. 

You also seem to be driving that gate to significantly lower than the -10V VGS the part is specified to. You are driving it down to 8V, which is -20V VGS. 

From the spec sheet, -20V is the absolute max value... that leaves you no room for variation in the 28V, part tolerances, and any other noise spikes. If you look at the other figures it's really defined at -10V, which makes that the manufacturers "normal" operating point.