I've tried the following, after looking through all forum posts and on the net, to no avail ...
- Nozzle checks - All fine.
- Head cleaning (multiple times) - Line still appears.
- Flushed print heads - Line still appears.
- Cleaned encoder strip (3 times) - No luck.
- Readjusted head alignment - No luck.
- Tried different paper - No luck.
- Promised Santa I'd be a good boy from now on - No luck.
