The classic usually only uses the left IRU for reference, try to manually switch from left to right inflight and have your ND in the lowest range. You will allways see a mapshift and a small course correction if in VNAV. So if the left develops an error and you switch to right one you might see a rather large mapshift indeed.
ANP should however return to normal once you switch away from the faulty IRU. However if you got a "VERIFY POSITION" scratchpad message you will have the position error and pointers on the ND for the remainder of the flight even if navigation performance is ok. At least thats how i observed it flying the classics so far (no GPS on ours except for EGPWS).