Mcbeth....
I'd just about bet a paycheck that the problem lies in over acidification of the curd.
The crumbly texture is a problem that I also fought for several years in my cheese making adventures and it was only after I purchased a Ph meter (and finally figured out exactly how to use it) that I started seeing this problem go away.
I've found that even with a Ph meter it is imperative that you keep on eye on the curd constantly.
Even five minutes of slacking off can make a huge difference in the type of cheese you end up making. Now if five minutes WITH a Ph meter can have that type of effect, think of what a timed recipe could do if it happens to be off, five minutes, here and there throughout the entire recipe.
I've decided that I simply won't make cheese without a Ph meter from here on out. Milk, starter cultures, etc. are not a negligible expense so I want to make the best cheese possible when I make the investment.
With a $30.00 - $100.00 investment you can verify that the recipe is either correct, or incorrect. In the long run, this can save you a lot of expense in milk, cultures, rennet, etc.
This is just my two cents but I honestly wish I would have figured this out a few years ago.
I sincerely hope this helps you out.
Dave