Propane Combustion in a Gas Grill with Irregular Flameouts
The brilliance of the orange in flares suggests suggests incomplete combustion of a carbon residue.  This can happen with a poor oxygen mix, but the blue in the core of the flame indicates that mix is fine.  

Also, on the base of the grill there is some charred residue.  

Based on the color, and the residues I expect somewhere in the flow you have some carbon deposits that are occasionally getting dislodged, causing the sputtering.