I'm having the same effect (i.e. wildly fluctuating SWR readings when using an ATU + wire antenna) with at least one QMX out of my "collection" and always wondered what the reason was. After a retune and/or leaving the HW tests function and going into normal transmit the SWR usually is where it should be according to the bar indicator. Definitely not caused by practice mode.
Haven't bothered to do a systematic analysis though, i.e. will the effect go away when plugging in a dummy load, different ATU, bands affected, stray RF, one or all of my QMXs affected, etc. etc.
Something clearly is amiss though seems to be an issue with very few of us, judging from the low number of responses.
A dummy load should of course always produce a rock stable SWR reading.
