Be it a cap leak or some other sort of leak issue, if your system isn't holding pressure sufficiently, you WILL notice the effect at the temp gauge.
If it is happening at idle or after shutting the engine off, I would point the fault to the reservoir cap...
...also make sure the cap is on SECURELY!!! I only had mine on 1/2 turn and dealt with the exact same issue for about a week before I realized the cap wasn't screwed on tight!
