Unfrequently

adv. (UN-¹ 11 and 5 b. Usually with preceding negative.)

1646.  Sir T. Browne, Pseud. Ep., 7. They like Judas desire death, and not unfrequently pursue it.

1674.  Boyle, Excell. Theol., 196. ’Tis not unfrequently so [prejudiced] by those, that mention him with an Encomium.

1794.  R. J. Sulivan, View Nat., I. 397. Systematic philosophy … is not unfrequently involved in difficulty.

1845.  Lindley, Sch. Bot., iv. (1858), 35. Flowers white, unfrequently pink.

1893.  Law Times, XCV. 56/1. Negotiations … not unfrequently fall through on some point of disagreement.