[1000mp] Old problem with my FT-1000MP

[Sergio R Rubio]

I am using a FT-1000MP for about five years, 95% on CW. Since about a year ago, the rig started to develop a problem.  When moving the main dial, it sounds raspy or scratched, either way, and sometimes, it stops receiving and transmitting for a few seconds.  This problem then disappears and the transceiver works OK for several days or weeks.  I have always kept it working with no more than 60-70 watts, even when using my separate amplifier
Last year, I sent it to Yaesu Calif. twice, and there they could not find anything, even when they said they had the rig always on for several weeks (only on ssb, I found out later).
The main dial mechanism was also checked, and it appears ok.
It is very difficult to locate this problem because it is highly intermitent, and happens, apparently, only on CW.
Any suggestions?

73, Sergio, KP4L