To add to bondo's theory of it being electrical, you stated that immediately after you shut the high beams off the gauge dropped. Your gauges work off of ground and a poor engine to chassis ground can cause this, ie, when you put a high amperage load on (high beams) they will pull ground from anywhere they can get it in order to light the bulbs, this can cause other grounded electrical parts to behave strangely. Your temp gauge simply measures resistance between the sending unit and ground and if that ground is compromised your gauge will give false readings. You still need to get the hissing/leaking looked at but at the same time make sure you have a clean, sound, ground from your battery to the chassis and from your chassis to your engine. Grounds cause more headaches in electrical circuits than anything I have ever seen.
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