My lo is the same. He was eating fine until about 7 1/2 months and then started doing exactly what you are describing. I mentioned it on here and came to the conclusion that because he was becomming more mobile at that point he simply did not want to be restrained in the highchair, and yes, I also found he was worse when he was tired.
It seems to have improved a bit now, a month on (although he's not taking anything solid at the mo as he's not well), I tried different things, like trying to distract him etc, but nothing really seemed to help, he just seemed to get better after a while.
Might not be the same for your lo, you may get some other suggestions, but sounds very similar to the problems we were having.