Dear Hans & Chia . . . here is the answer from Rob Simpson:
Hi Jack, Thanks for the great info. The first thing I suspect (if no driver or RIP settings have been changed) is that there is now "chatter" in one or more of the carriage bearings. Sometimes you can feel this by moving the carriage left and right with your hand. Sometimes you just can't feel it however. In the past I have experimented by attaching a weight to the carriage - say a roll of quarters firmly to the head in an attempt to dampen any velocity changes brought on by a bad bearing. In uni direction printing this should work for you more than bi-d. This can sometimes be a temporary fix. What you describe sounds very much like this is happening. The bad news is that they made the carriage so to replace the bearings is a big job and the entire carriage is exchanged. Maybe not cost effective on an older printer..... The new printers 7900/9900 printers are really good. there are many benefits.
Perhaps this might 'solve' your problem but also, like me (who has had zero problems after nearly two thousand large prints 20x24 and larger), the printer is now dated and the new one's Rob mentions are really superb.