fabsport,
I've noticed EXACTLY the same phenomenon with ANY beta blocker (I've treid several). Perhaps a little less Toprol is the answer- tell your doctor when you next go in. Fifty BPM is perhaps a bit toolow , but 60 is just fine.
p.s. I don't think it's anything to worry about- the slowing heart rate is one of the benefits gained from beta-blockade.
(Needless to say, I'm a drinker too)