Compare value from same table, same column, but a differnt time period

I am trying to catch time periods that have a illogical value. same table, same column but different date. How would l write the query? in the below example, the odd line to catch are 3/31/12 & 3/31/11 value of 99.2 & 92.2

Any help is appreciate.

comcode adid date yearendmonth shares
100001 01W 9/30/2012 12 1000.689
100001 01W 6/30/2012 12 1000.689
100001 01W 3/31/2012 12 99.2
100001 01W 12/31/2011 12 991.672
100001 01W 9/30/2011 12 991.672
100001 01W 6/30/2011 12 953.488
100001 01W 3/31/2011 12 92.2

Thanks!