Hans,
I believe I have (multiple) examples of wave gaps on the following years:
1966
1967
1968
1972
1973
1974
1975
1976
1877
1978
1980
1981 (seems to go hand in hand with the 3 & 1/2 claw)
1982
1994
1996
1997
1998
1999
2000
2004
2005
2008
2012
2014
Many of the earlier years are on coins that are at the lower end of the grading scale so can't be 100% certain but as I have more than 1 for each of the years I am leaning towards the occasional existence of gaps.
As well I think I have single examples for 1970, 1971, 1979, 2001, 2006 & 2007 but with wear and tear a bit less confident on some of these.
Pedro