Tuuk wrote;- I have the MFJ 989C, and I am noticing that I can tune
antenna to low SWR
but I also have noticed that the SWR often was jumping

I had a similar problem when I bought a second hand 949C manual tuner. I
found an earth tag to the metal case was not making proper contact, I
cleaned it up put in a new bolt and washers and it's been fine since,
Hope you find your bad contact.
73 g0wal