Overflowing Resevoir

[Drag0na5h]

About 2 years ago a new motor was put into my Supra (7mge) and since then noticed difficult starting no matter what time of day. Also I noticed white smoke on start up after installing an aftermarket exhaust. It's possible I had this white smoke problem before with the stock exhaust, but I notice it a whole lot more frequently now. Eversince summer began about two months ago I noticed that after driving home from work (15 min on Freeway) my car would immediately spit out hot coolant from the overflow resevoir tank after turning off the engine. The weather here has been around 90-100+ and my cars thermostat reads normal right below the middle mark. I recently changed the coolant temperature sensor and my oil. I didn't notice any milky residue in the oil then or now. I'm constantly filling up my resevoir tank to the full mark almost everyday (if I drive far). What could be causing this?