I finally found the solution to the problem that was causing bad TX IMD on ten meters and which drove me almost to the point of insanity!
I had taken all the radio equipment off my shack desk, and was slowly replacing and reconnecting it, one piece of gear and one cable at a time. Each time I added something, I checked the PSK31 IMD on ten metres. Nothing seemed to make any difference. I reconnected the 2m transverter to the K3 and everything checked OK, with IMD of >-30dB. There was just one last thing to connect - my 2m Slim Jim. As soon as I connected that, the IMD meter started beeping and reading -14dB plus or minus.
Disconnecting the 2m antenna cable didn't fix the problem, though, although it made a slight improvement. Everything had been perfect, and then reconnecting this one cable had wrecked it. I was tearing my hair out at this stage, as you can imagine.
Then I noticed that moving the unconnected antenna cable about drastically affected the IMD. Grasping the cable in my hand improved the IMD substantially. Gradually it dawned on me that this antenna cable must be re-radiating my 10m signal back into the shack in such a way as to affect the K3 transmitter. When I first disconnected it and moved it out of the way, I had quite by accident found a position where its effect was neutralised.
When I made the Slim Jim I had hung it up in the attic only a few feet from the 10m dipole. Since it was cross-polarized and for a completely different frequency I did not expect that either antenna would have any effect on the other. Both seemed to transmit and receive OK and the SWRs were unaffected. But obviously the Slim Jim was picking up the 10m RF and causing trouble. I had never imagined that the 2m antenna could be the cause of my 10m problem and so I wasted most of the holiday weekend chasing red herrings and running up blind alleys trying to solve it.
This morning I went up into the attic, took down the Slim Jim and coiled up its feeder cable. The loss of 2m FM is no big deal for me. I could probably disguise it outside if necessary, but there is so little 2m activity here anyway that I really can't be bothered.
Just for the hell of it, while I was in the attic I also removed the old 10m dipole and added two 8ft. long elements to my 80/40/20 dipole, fanning them away slightly in a very shallow inverted V. This gains it a bit more height and eliminates the need to manually switch between the two dipoles.
You can get great results with attic antennas, but it's easy to get carried away. You can't just keep stuffing more and more antennas into the loft space without finding that they interact in some way. Between my now 4-band dipole and my MFJ magnetic loop I can use about 80KHz of 80m centered around 3.590, plus all of 40m, 30m, 20m, 17m, 15m and 10m. That's not bad for a situation that would have many people giving up the hobby altogether. I will just have to not be greedy for even more bands in future.