I have that problem too. I started cycling my weight over 4-6 wk periods. The heavy end of the cycle is something I can only get 4-5 reps with 2 wks in a row. Then when I hit that peak I cycle back the equivalent of about 10-30 pr wk. for the number of wks. the cycle will last. Say I'm heavy for 5 reps at 405 on my squat. If I intend to do a 6 wk cycle at 20lbs pr wk, I'll take the weight back to 285, ( 20lbs x 6wks = 120, 405 - 120 = 285 ). So at that point my reps will go up where they may and I will do sets to failure, NOT FOR REPS. Always do sets to failure after warm up work instead of for reps no matter what the weight is and you will grow much faster, guaranteed! Another rep is better than another set where intensity is concerned and that's what makes you grow.
My point also is that the cycling gives your skeleton time to recover on the light end from the heavy weight and by the time the cycle gets heavy again you feel strong w/ much less pain.