Another busted reservoir tank

[TonyVA]

Ok, I am at my wits end! Driving yesterday (2001 X5 4.4i) I saw smoke coming from the engine compartment, pulled over and shut it down. Not surprisingly the seam on the reservoir had split. Towed it home, ordered the new one pulled it into the garage and installed it...20 minutes being careful with the hoses. Now here is my challenge...bled the system with BMW anti until there were no bubbles. I will do it again in the AM, however, where I am out of answers is the temp gauge stops right about where I figure the temp should be and then blasts over to the RED! Take the fluid temp and I am hovering around 170 F. So why is my gauge stopping for about 1 second at the correct temp and then slamming to the RED? Need help, anyone?