I'm quite sure that is supposed to happen. It should stay in the middle. That is the ideal temp. When you smash the gas, the engine will get hotter, but will be quickly cooled off by the cooling system. I wouldn't worry about it unless the needle is at one of the extreme ends.
<span style='font-size:8pt;line-height:100%'><span style='color:blue'>1998 BMW 323is Coupe, Black
18" M3 Hyper Silver Rims
2 12" Rockford Fosgate Subs; Sealed
1.0 Farad Capacitor
800 Watt 2-Way Rockford Fosgate Amplifier</span></span>