When I go down hill on the bike the heart rate shoots up to 199-210. There is no way this is correct. anybody else have this problem with the forerunner 305? What do I need to do to get an accurate heart rate with the 305?
IME, it’s from the sweat drying and not having a good electrical contact. Going downhill
your strap tends to “gap” anyway, and you’re drying out all the sweat.
Cinch up your strap more, and pedal occasionally. 
-Jot
I’ll give it a try.
It’s not the 305 per se, but rather the strap. In short, certain things contribute to the erradic readings - in particular, at high speeds when on the bike. To fix this you can twist your HR strap around a bit towards your side (or even back). You can also apply copious amounts of HR gel to it as well, which can sometimes help.
I wrote up a bit long thing a while back on all the things that can cause whacky HR readings, and how to fix them. May be of interest.
Normally though, I just lick mine. But today for example, I did a lick and twist to resolve the issue as it was pretty dry around my area this morning.
When I go down hill on the bike the heart rate shoots up to 199-210. There is no way this is correct. anybody else have this problem with the forerunner 305? What do I need to do to get an accurate heart rate with the 305?
It’s not primarily the strap or the skin contact - it’s your floppy, dry jersey. Try a tighter jersey or something tighter under, next to your skin. You can also try to squirt water on your jersey to see if it goes away.