I had a similar problem when my lifts started getting heavier. Once I could squat 225+ I warmed up less gradually. Maybe try warming up at 95, 135, 185 then working sets. Once you get to 250+ drop the 95 warmup set and go 135, 185, 225 then working sets. Hopefully that can help. I realized that I lost focus on tightening up my core when the weight was too easy to lift then when the bar got heavier I wasn't focused enough to properly execute.