Garmin Foretrex 601

I have two Foretrex 601 s

The first runs successfully on GPS and Glonass

The second will not and loops out with (Reception lost)

It will as previously mentioned run on GPS as a stand alone with no Glonass

There is a brief mention to the problem on this forum but not explanation

I walk in thick rain forest and need as much reception as I can there I need to set to "GPS and Glonass '

My question is how do I get the second Foretrex to accept the Gps /Glonass setting without rejecting it?

I also own a Etrex 20X,Etrex Touch 25 ,64ST map 

The Foretrex 601 is the only unit where the downloaded track will jump around and invert 

My question is 

Why is the Foretrex 601 display unstable where as the displays on the other units are rock solid and stable?

Thank you