OK, I think the constant shift with just the lamp confirms that there was some shift in the spectrograph (It is very small. I think 3A equates to about 5um or about a pixel). The changing temperature might have been a factor. With the ALPY I usually take a lamp spectrum each time I change target during an observing run even though I normally dont see any shift during the night. (With the LHIRES I see a lot of movement so I take a lamp spectrum at start and end on every target and every half hour or so during long exposures but it is much less stable thermally and mechanically than the ALPY)
Cheers
Robin