Dan Gibson EMP p650 issues

["samoyedd" samoyedd]

So I found some old parabolic mics laying around our Bio dept. stockrooms, and 
they work to an extent.  The issue I am having is that after using them for a 
minute or two, they cut out or start making really strange sounds.  However, if 
I then turn the amplifier off for a while (1-5 minutes) they work fine for 
another couple of minutes.

So my question is then, is there a fix to this? I hate for equipment like this 
to just gather dust, but as is, they won't be very useful in the field. 

Any light you can shed would be much appreciated.