you need to bleed the cooling system. cant tell you where your bleeder valves are on that engine but they will be easy to spot(look for the highest point in the cooling system.) take the radiator cap off, start the engine, when the fan kicks on open the bleeder valve. let the coolant run out til there is a steady flow with no air bubbles or spray coming out. my s2000 has had that problem a few times whenever i had the cooling system open or changed something. sometimes i have to bleed it a few time to get all of the air out. if you have air trapped in the system the air pockets will collect and not the the coolant flow a complete loop air will get trapped behind the theromstat and not allow it to heat up and open and close like it should. is your car running hot? i know the gauge will still be showing cold but are your fans coming on? is the engine itself getting really hot or boiling over? give the bleeding a try first. wont cost you anything. also when you have the engine running, rad. cap off and bleeder open thats when you will want to top off the coolant level. hope some of this was a help to you. if not sorry. good luck with that. pm if you have any questions