I just had the problem the OP described, on my Garmin 64st.
* Hard reset did not solve the problem.
*Turning off GLONASS immediately solved it.
This seems to be a very intermittent problem with GLONASS; I've been running GPS/GLONASS without a hitch for about a year, until my GPS started consistently giving me "Weak GPS Signal" no matter where I was. I'll try turning GLONASS back on sometime and see what happens.