yeah its and electrical prob. seen it many times before, usually increase in gauge temp corrisponding with boost, but may show gauge temps. inaccurate in either too cold or too hot, ...i used to work for toyota as a technician, and i own a 450 hp monster myself. ive seen this prob- even on my own car a few times. lots of techs change temp sensors and dash gauges... my gut feeling is it is in the printed circuit board behind instrument cluster. easiest fix- buy good quality afermarket gauge ( autometer cool. temp etc etc...)
pull out dash, ...install new gauge and sensor and get modern, accurate readings without all the guess work- very important if running a modded turbo inline6 and you cant replace that piece of mind not having to think your "head gasket is blown" ..which is said WAY too much in any discussion reg: supras....supra engines are very resilliant, most blown HG probs are poor maint. and neglect, or potato chip begs between rad and intercooler( no joke)