Why does my Cadillac 2000 DHS temp move up and down?

[Newone]

It only moves when i am stopped at a light for a minute or so. It goes from half/middle up a 1/8 to a 1/4 past the middle some times reaching the line before the red. Hear is the kicker it does not over heat. But it does spit coolant out of the overflow hose when it gets close to the over heat point. It goes back down when you have high RPM s.

What i have done.
New Hose (OEM was aged badly)
New Thermostat (it did this before so i had gm put this new one in)
New coolant (flush and fill plus treatments With Global)

Here is what i know
It dose not do it on the Highway ill drive 4 hours or better it never moves. If it did it on the highway it would be a head.

DEX COOL SUX

What i think
It could be the original water pump is too old cause an improper or uneven flow of the coolant.

Thanks in advance for you time. I am no tech but worked at a shop and this is where i got most of my information from.

[NicN]

sounds to me that your fan isn't working do you have one if its a thermo fan maybe there's a fault in the electrics or it maybe set at wrong temp your gauge is going up when you come to a stop cos there is no air going through your radiator that's what the fan is there for to keep the radiator cool

[OldManDirt]

I too suspect the electronics, in specific the fan relay. Try turning on the AC and see if they (or at least one) of the fans kick in. If they don't have the relay checked out.
It could be the tempsenor for the ECM has failed.
But in most cases it is the relay, not the sensor or the fans.

[OldManDirt]

I too suspect the electronics, in specific the fan relay. Try turning on the AC and see if they (or at least one) of the fans kick in. If they don't have the relay checked out.
It could be the tempsenor for the ECM has failed.

But in most cases it is the relay, not the sensor or the fans.