IBM 4234 dot matrix

I have a customer site that has the problem that the print gradually moves up the page by about 1/4 line per print job. I checked it out myself and the symptoms are these:
The spacing does not change within a job and so you can print 40-50 pages and see no change.
The spacing does change with each new job and the print moves up the page slightly. After about 10 jobs the print has moved up 2-3 lines. This also occurs when doing test prints from the front panel.
The configuration seems to be proper for the forms they are using. (Green band wide paper)
I have no manual for this printer. (I do have it for the 4230/4232 but these seem to be different enough that the instructions are not the same.)
Any suggestions?