I am having the same issues with my Chargriller. Really caught my attention when I tried to smoke a turkey for Thanksgiving and I just couldn't for the life of me get that last 15 degrees to finish off the turkey. I had to finish it in my oven. Boy I was really frickin pissed off about it and couldn't figure out what the issue was until I found this site and your thread. For now though I have been using the blanket method where I double fold four of them up and layering them on top of the cooking chamber. Believe it or not ( I tested it out yesterday before cooking in it ) I gained 40 degrees in the chamber by doing this. So anyways what do you all think about trying a commercial fire blanket. They are pretty thick and used to put out a fire on a cooktop so why couldn't we use it over the cooking chamber for that extra insulation. It should really hold the heat pretty darn good. I am in the process of researching the cost getting one but I have been out of work for a while and my funds are pretty low right now.
Let me know what you guys think about it.