Lotus Elise S2 (K Series Engine) Over heatingTemp gaude up and down like a Yoyo?

I have just had it back from the garage yesterday as it went in for some work it started running very lumpy one day , I thought it was just going to be an easy fix as it was always running spot on. It has been there for 4 months!! and finally picked it up yesterday.After a long string of testing on the engine etc he said he thinks it was the ecu so he changed that but that one got wrecked due to a power surge ? so he advised me to get another one a (Emerald ) aftermarket one and i agreed that he may as well do some other minor work to tune slightly which are ( Decat pipe and a TIG Air filter) so with the emerald it should be a great accompliment . Anyway i drove it home from the garage to find out the tept gauge is up and down running from 80 degs to 119 !!! and some steam ????? I was not happy . It was sticking to 97 most of the way and popping up over the 100 then coming down (The Fan kicking in) any ideas what this could be ?HELP

Lotus Elise S2 (K Series Engine) Over heatingTemp gaude up and down like a Yoyo?
Thermostat is not working properly to maintain temp. needs to be replaced. should be an easy fix.