If your crush looks OK, grains are cracked & the inside is accessible, then one thing to consider is making sure your your thermometers are accurate. Check them in ice water and boiling water- they should be 32 and 212 if you are near sea level. Could be you are mashing much higher or lower than you think? Another thing to try is mashing for 90 minutes one time to see if your lower efficiency is coming from lower conversion during the mash or lower extraction of the sugar from the lauter. If a longer mash yields higher efficiency, you may not be getting complete conversion for some reason (crush, temp, pH?). That said,I would rather have a consistent 50% efficiency and spend a couple of bucks more on grain, than one that is all over the place and makes hitting your numbers a crapshoot.