Further information:- The jumping never happens the first time I record. Only afterwards. Recording still shows as set to begin at start of song.
Now if I restart the problem remains. Also the recording of my voice is now totally distorted even though volume levels are showing as within safe limits. It's a deep booming sound - the same has happened on one occasion to the drum track. I have re-installed BIAB and Parallels to no avail.
I do hope I'm missing something really obvious....