60HP 4stroke overheating

[Lollipop2]

Hi all. I'm new to the forum and hoping someone can help me. I have a 60hp 4stroke outboard that I purchased from a dealer second hand, with only 100hrs on the clock. I used it twice then it sat idle for 5 years due to boat and trailer issues. During that time it was run to operating temperature every 3-6 months using the muffs. It ALWAYS had a very healthy jet of water from the tell tale. Then my battery died and it wasn't run for 12 months until 2 weeks ago. Within a couple of minutes the overheat alarm came on and although there was still plenty of water coming from the telltale, it was nowhere near as good as it used to be. My first thought was the impeller. I put a new impeller in on Friday. Tried the motor, it overheated again. So I checked the thermostat. It had a lot of corrosion and it was stuck closed, so I cleaned and freed it up, put it back, tried the motor, still overheated. So i took the Stat out, put the housing back on, ran the motor at varying rpms for 15 mins without it overheating. I tested the Stat in a saucepan, it opened at 60 degrees celcius and was fully open at 70 degrees. So my question is, is the thermostat opening at the correct temperature? Had anyone else had this problem before? What else can I check that might be causing this overheating? I have checked the oil level and fuel filter for water, both were good.