Heiz High Z 1000 T Z axis rigidity

Hi,
New to your forum and looking for some solutions to a problem I have...
I have owned the Heiz since 2009, bought it new from Germany way back when.
Has not been overly used but am now getting more work in that the machine can do so hopefully it will earn its keep.
Mainly been using it for sign work in wood but now moving into non ferrous machining of sign plates for vintage vehicles etc and getting some nice results.
However, even though the machine and spindle should be man enough to cut some quite deep passes I am finding that the instability of the Z Axis causes more than enough problems and was wondering if anyone has gone down the route of changing this to a better designed unit or is the issue inherant in the Heiz build itself...
Any suggestions / ideas would be gratefully received.
Thanks
Nigel