I have a like new 1998 Saab 900s convertable. My husband recently sprayed and cleaned the motor with the engine off. When we turned it on the check engine light came on so we took it to the dealer to get the light off.   I have noticed that the temp gage will some times register in normal range. Then after a few miles it just drops all the way down as if not registering at all. Is this a normal thing or can you tell me what causes it and if it is anything to be concerned about.

Optional Information:
1998 Saab 900 s convertab 2.3l

Already Tried:
not any thing