I've also had 2 99s now that have needed the tension spring on the bobbin case replaced. That will make you pull your hair out. In my case , the tension fluctuated and was very hard to pin down. I suspect it's from trying to sew with thread that's way too heavy. It's not a bad idea to clean out the tension area of the bobbin case too. I regularly find tiny hardened duct bunnies in there.
Which tension assembly do you have? The one with the external spring, or or one with numbers? I have to clean up a 66 this week, and can post some pics if needed.