Thanks everyone for all of the suggestions. As soon as I get some time again to get in my shop, I'll try to start from the beginning with 80 grit and set the feed rate to 40 (out of 100) with a very light pass and then work my way up from there.
The last few time when I was having problems with the burning, I thought I was feeding it too fast (40-50) so lately I slowed it down (20-30) but still having the problem. If I recall correctly, the first pass is usually ok but then as I keep running the board through the sander, it starts to get worse.
I'll report back when I get a chance to test out my technique to see what is going on.
Thanks again and I hope you and your families have a very Happy Thanksgiving.