Fuse box question
 

In my basement I have a fuse box with 60 amp service. One of the (not
used) 20 amp S-type fuse assemblies is a tad loose. Using a tester, it
is sometimes hot, other times not, depending if I giggle it. The fuse
itself is new and tests fine. From what I've read I cannot unscrew the
female S-type thread ring. How can I fix this problem? Is it possible
to just replace the (duplex) fuse block? I plan on adding a new
circuit to this fuse location.

Thanks, Dan