If you are writing job letters and revising résumés three or four afternoons a week you might be applying for too many different kinds
of jobs. A more focused job search may be a better approach. Obviously, I know nothing of your background, but you are probably not qualified to do ten different things; it is likely that you are very qualified to do, say, 3-4 different things (and your résumé shows this). Accordingly, you should be limiting your job search to those four types of positions. Applying to those other six is a waste of your time.
I will defy conventional wisdom here and tell you that you do not need to revise your résumé for every single job. You should probably have perhaps three versions of your résumé: A, B, & C. Similarly, you should have about 2-4 versions of your application letter (1, 2, 3) each accentuating different parts of your experience. When you see a job that is consistent with your background and skill set you should immediately think to yourself, “ah, for this job I will want to use résumé A with letter #3” or résumé C with letter #1, etc. Of course, you will want to tweak each letter accordingly, but that is something you can do in a matter of minutes. In my view, spending “three or four afternoons writing application letters and revising résumés tailored to each job” is an inefficient use your time—especially since so far, you have not heard from anyone.
For strategies, read what I say here about becoming Person X. http://chronicle.com/forums/index.php/topic,86688.0.html
There may be something substantially wrong with your letter(s) or résumé(s), but because you’ve been working with them so consistently you may not be able to see what the issue is. Have them reviewed by a critical third party to help you identify any red flags. Ideally, this reviewer should be someone who sees lots of résumés routinely.
The ramp up faze of a search can be taxing, but once you have solid application materials at the ready a job search should not be exhausting, and certainly revising letters and résumés should not be consuming so much of your time. Your time will be better spent networking and becoming a known quantity on the campus(es) where you hope to work.