[llama]

Hi David and welcome.
It appears you are using a tape that has been recorded on before and that the original recording is half-track. If that's right, do you know the age of the tape? Newer tapes have a magnetically harder formulation and it may be that your machine doesn't have the erase power to fully wipe it before the new recording goes on. An older tape may be more suitable.

But maybe the erase head is just mucky.
Graham