Have you checked the basics?
have you thought about checking bike set up etc or getting a chiropractic exam to see if it's actually something to do with eg nerves being pinched - maybe only on hotter days because then blood vessels etc swell and may be more susceptible to problems?
what about dehydration - maybe you are actually dehydrated before you start, and riding in the heat makes it noticeable?
I always like to look for a cause so that I can resolve that before I look to treatment of symptoms.