Not sure what your fuel pump assembly set up is, but I had this EXACT issue with the OE fuel pump assy. (everything that hangs from the hat). My system was all OE and running gasoline. I replaced the pump and had same issue. After I replaced the entire assembly the problem was solved. The fuel pressure regulator is located within the plastic assembly somewhere so I believe it was the problem.
If you still have the assembly and the regulator contained therein I'd lean toward that. I didn't see any notes about testing your fuel pressure so that may be your first check.
FWIW, I had a return E85 system on a previous roots blower + nitrous V8 platform in S. Florida heat and NEVER had the issue you're experiencing. I'm not saying it's not possible, I just didn't experience that in heat. Additionally, if it's happening "all of a sudden" and not during previous heat spells, I'd look beyond the weather.
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