Unfrequently

1813, Jane Austen, , Chapter 30:She not unfrequently stopped at the Parsonage, and had a few minutes' conversation with Charlotte, but was scarcely ever prevailed upon to get out.
1838-1839, Charles Dickens, , chapter 4:Near to the jail, and by consequence near to Smithfield also, and the Compter, and the bustle and noise of the city; and just on that particular part of Snow Hill where omnibus horses going eastward seriously think of falling down on purpose, and where horses in hackney cabriolets going westward not unfrequently fall by accident, is the coach-yard of the Saracen's Head Inn;
1841, James Fenimore Cooper, , chapter 3:Hurry had all the prejudices and antipathies of a white hunter, who generally regards the Indian as a sort of natural competitor, and not unfrequently as a natural enemy.

This was the more common form in the 19th century (vis-Ã -vis infrequently).