So let's get down to elemental diagnostics...
Is the QCX 20 new? i.e. Has it ever worked?
Does it only add dits AFTER another dit? Dahs after other dahs? Or seemingly random?
Have you changed anything in the environment?
Might there be wire whiskers in the 3.5mm plug?
Any routing issues with the key cable? Strong magnetic fields? (The circuit relies on internal pull-ups in the processor, so it isn't "brick s-house" immune to induced noise).