If your solvent is boiling you are way too hot-- I've never seen any instance where that kind of heat is needed or recommended-- not to mention the serious hazard of super heated extremely flammable vapors under pressure.
I don't usually go much above 50C myself usually (I use a sous vide heater to heat my jar in a vat of water-- with the solvent added. Very accurate, safe, and great overall solution for targeted-temp long-length heating/extraction), and get extremely effective extractions.
I also use VM&P Naptha, and have never seen it boil-- even at 60C in my sous vide for 1,5 hours.... so I'm not sure why yours is reacting that way.