Hi Tom and William,
It isn't all the time, just periodically.
I guess it could be RF getting back in, so I will try using less power, but I don't have issues with RTTY or PSK software, only WSJTx and JTDX.

I do keep a very close eye on the time sync, and refresh that quite often.

Also, it doesn't seem to matter how many signals are on the band. I could understand if the band was packed with really loud signals, but it doesn't seem to make any difference.

All very puzzling...

73 de Phil GU0SUP