Hi Steve: 

Well 1ST up, my tuner is very old. I bought it last winter from a Guy who was the 3RD owner of it. So, its probably in access of 15 to 20 years old. 
It probably needs a good cleaning no doubt. There was no fluctuations in the SWR or full power out when the antenna was directly connected to the 570. I got my XYL to look at the display for that one. 
The flux only occured around 3.79MHZ with the MFJ-989  and I had to rotate the roller inductor knob about 10DG clock or counter clock wise to keep the SWR flat. 
So, this is how it goes. I tune for the low SWR, unkey, key up to check and the SWR would be up to about 10/1. I'd key again and rotate the roller inductor again to lower the pitch and SWR, take a reading and it would be flat. I would receive for about a couple of 2nds and want to take a reading so would key up and boom, the SWR would pop back up again. 
All other bands perfect. Almost something might be arcing in there but don't see any sparks or hear any crackling or smell any smoke. LOL 
The thing is, if it was the roller inductor, wouldn't that occur on all bands or it depends on if you happen to hit that spot that might be very dirty? The tuner is rated at 2KW SSB/1500W CW, my rig is rated at 175W CW, so don't think 75/80W is too bad. 

73: 
Michael DE VO1AX