The Plus should handle those ambient conditions ok. Perhaps you have poor connections that are particularly sensitive to changes in temperature. Common culprits would be RAM, as well as other socketed items (ROM, IWM, etc.). It may be worthwhile cleaning those contacts and trying again. For the socketed items, just (gently) remove and reinsert the chips. For the RAM, clean the SIMM contacts with a pencil eraser and reinstall. These simple things might restore stable operation.