First, check to make sure you have a mostly blue flame all around the ring. You need to leave the bottom vents open but you can close the top vent to get a little higher heat (unless the tab is bent, you can't and should not close the top vent all the way. If it is windy out, you need to provide a wind break. A fire resistant material (welding blankets work fine) around one or more sides will help retain the heat - you can go all out and do an enclosure like I did for my GOSM - see pic below.
If you repeatedly have good temps for 20-40 minutes and then the temp dips dramatically, you may have a problem with the regulator and/or the tank. I had problems in the cold with some tanks that worked fine for awhile but then iced (the inside) of the line and restricted the propane.
My GOSM with insulated cart - tested in 20 degree temp and ran for over an hour at 400 degrees.