I had this same problem first time I squatted heavy, and like you I've had intermittent low back problems over the years (I'm not yet 00). I narrowed it down to 0 issues: tight piriformis (especially left side); and tight hamstrings. What I've done to solve it:  0. stretch piriformis and hamstrings and warm them up for a good long while before squatting or deadlifting  0. keep lower back arch tight and in the correct position - maintain the same curve you have when standing. This involves keeping those muscles as tight as possible. I find this limits my ROM - can't quite reach ass to grass. That means when I WAS making it ass to grass I was rounding my lower back which is bad, bad news for your lumbar spine.  