This has happened to me many times. It took a while to narrow down. I had at my disposal a thermographic, or IR, camera to help too. I have found it to be an issue with the HIPS degrading as a function of heat AND time. The longer it’s at high temperature, the more it degrades. This happens if you print at 240C at 2mm/s as well, then you’ll see the yellowing at layers which are printed more slowly than others. The higher the temperature, the less time it takes. It seems to happen more readily with the white HIPS. The other colors have additives which change the behavior. At first I thought it was my build plate, which although it does have variations of around 25-30 degrees C (TAZ 6), the main problem was just temperature VS time. The design doesn’t help though. The TAZ 6 has a single temp sensor on the left third, about mid-way to the rear. If you put a part on the sensor, you are in a much cooler region than if you put it on a hot spot. Lulzbot should’ve done a better job designing a heated bed because your printing results already depend on a large variety of variables, now you have to throw “where on the bed you printed” into the mix. If your print is large, you can forget it. HIPS will just degrade, the melting point will drop, and it will no longer even serve the purpose of a good raft. I’ve tried many brands, even the Lulzbot suggested “Village Plastics” HIPS on their very own website.