Mitza....(I read your msg 3 times...hope I got it all right).
If cars are stationary, then they will heat up very quickly (the reason why dyno's have massive industrial cooling fans). The temperature 'outside' really doesn't matter much as there is no wind to make the hot air flow out of the engine bay.
To move onto your 2 scenarios:
1) Cold night, you thrashed your car, then stayed staionary. The fact that you had thrashed your can basically negated the fact that it was a cold night, the engine was really hot, therefore temp went up quickly when stopped in driveway. Once you went again, movement caused air flow through engine, therefore temp went down.
2) Hot day, no thrashing, stayed stationary. [Was this after a drive or not?] In anycase, the engine was cold, therefore without heating it unduely (by thrashing or just driving) it will take a fair while to cause a major rise on the temp gauge.
Remember, the difference between a hot and cold day is really almost insignificant compared to the combustion temp in engine, therefore being stationary in a already well and truely 'hot' car is going to send the temp gauge up.
GliTimmy......$170....you've been had! It is really easy to flush it out yourself, and costs about $20-40 for coolant.
Also, sounds like you have had your car refilled with shite coolant that is not very efficient (cheap), therefore is worse at diffusing heat than the coolant that was in your car before (my guess).