In this one case, the primary key solved my problem. But I wanted to understand what was happening with these indexes for future reference. I removed the primary key and all indexes from the table. The table has 70,737 rows. I used the same values for each column in all searches.
Columns used in seraches are as follows.
This is because all rows in the table had the same year and therefore the index was the same size as the table. When year was move to the end of the index, the performance improved significantly. This would seem to indicate that the order of columns is important when creating an index with the columns having the most varied values placed firstg and the columns having the least varied values placed last. Indexes, even if non-unique, do make a performance difference. If an index is defined on multiple columns and the where clause contains all columns in the index, there is a performance difference if those columns in the where clause are not in the same order as the index.