Mine go through spells - I'll replace 'em a few times in one year, and then get 2 whole years of life. They aren't that expensive, just a hassle to change so I don't really worry about it.
I got the same problem, I gave up replacing them. I do not know why this is happening.
Ciao Forza America!!! CT in the House!!! 98.5 Ford contour SVT Alpine head unit, mbquart separates up front, 6x9 pioneers in the back. Running 500whats fosgate to two 10 kicker subs at 2ohms. Running 50x4 fosgate to the front and back speakers.