additional info:

on ssb i hear my distorted voice in headphones when transmitting (monitor is
off)

MAc

In my experience, I had somewhat the same problem. Once the
operating chip starts to fail, you will loose bands one by one starting
from the highest band like 10 meters. Then you will loose 15 meters,
20 meters, then 40 meters one by one. Send it to a authoized ICOM
repaid center and they will restore the missing bands as I did.
Later found a hame in Italy that was marketing a new ramboard that
would last forever and you would remobe the old ramboard and install
his. After that you cant loose the operating system. You could loose
the "memory" chnls as they are a seperate entity and require a good
backup long lasting to retain the "memory" chnls only. So far I have
had no further loss of bands. Nor have I lost any memory channels and its
been about 10 years now. I dont remember the Italians name but think
it was Bernardo and he was a ham operator and marketed those rplacement
RAM boards world wide. Stictly for the ICOM751.
Joe KH6JF