temperature guage problem

[71lwb]

i have a 71 chevy with guages. i cannot get the tempurature guge to function properly. i have replaced the old broken guage with a reproduction guage. as the engine warms up, the needle goes all of the way over to the stop just past the red. my mechanical aftermarket guage verifies that it is running at 180. i can pull the cluster out and ground the guage out with my self powered test probe, but as soon as i restart and rewarm the engine, it pegs again. i have read on previous threads, that there are variable resistance senders. i am leaning toward this being my problem, but i would like some opinions on this problem before i do any more to it. if anyone believes this to be my problem, a proper part # would be very helpful. any help on this matter will be much appreciated. thank you