At about 80 C enzymes start to denature, and no longer work. You end up with little fermentables and a lot of starch.
A rough rule is:
- 62°C for maximum fermentable sugars, mostly maltose (alcohol in finished beer)
- 72°C for maximum not fermentable sugars (body and residual sweetness)
- 82°C for mash out, and to stop any conversion
If you mash at, say, 65°C, and you only sparge at 82°C and it's bad, but was OK when you did sparge at 78°C, it means you should simply mash longer. Your previous process didn't break all enzymes before sparge, so starch could also be converted during sparge, filtration, and before boil. Now it cannot, so you need to make sure it's all converted before sparge. Consider iodine test, or two hours mash, with generous stir every 20 minutes.