Dave,
Something doesn't sound right if it's wanting to dip.
What lathe & QC?
Sounds as if it isn't too well bolted down.
My first QC was on a 'small' Emco V10P with latest on a Warco BH600G and on no occasion had I/have I been in the least unhappy with them, quite the reverse.
I recommend a QC without reservation!
Geoff – Still haven't fixed the lathe 