Hmmmm,
It's possible that the old driver tube going out. I've seen it happen with a Drake T4XC I had Years ago, and an FT-101B I had in college. I guess for now, just keep an eye on your FT-101, to make sure it doesn't start misbehaving again. Oh yeah, and you probably already know this, but in case you don't:
1. Don't try to run more than say 25W of carrier for AM with your FT-101, otherwise it'll not only distort the signal, but also burn out the finals over time.
2. If you can, get your hands on the AM filter for it. At 2.7 or 2.8 kHz of receive bandwidth with the stock filter, AM will sound kind of muffled. 5 or 6 kHz will sound better. INRAD makes a filter that should do the trick:
http://www.inrad.net/product.php?productid=112&cat=27&page=173 & Have Fun,
Ellen - AF9J