Bad echo is distracting

SNOM 370 7.1.33
PolyCom SoundStation IP 7000
Digium TE121B connected to a PRI

We are experiencing echo in many circumstances…

when making extension to extension calls (from SNOM to SNOM)
if either party uses a headset there is a noticable echo.
also, calling an outside party, there is an echo, albeit not as distractive
even when using the handset
the Polycom has a very bad echo, though it is not consistant, at times it works great… though it does seem like after some amount of time on a call it starts to echo. Sometime the echo starts immediately

Any ideas? what can I try to eliminate the echo?