The symptom you describe exactly matches my wife's car. If you get cabin heat only when moving the problem is your aux coolant pump.

The pump is a brushless DC motor and internal failure of electronic parts make it stop running.

The pump is right below the MAF.

You can likely kick start the pump with a mechanical tap; ideally in such a way that twists the motor a bit.

https://uploads.tapatalk-cdn.com/201...7791bc7e33.jpg
Healthy motor winding signal east-west.

https://uploads.tapatalk-cdn.com/201...80860f846f.jpg

Unhealthy signal north-south.

I took these readings off a failed aux pump. I didn't have enough time to determine which part had failed. The faulty signal interfered with the starting torque. I was able to make the mirror spin every time if I gave it a starting twist

I noticed that my X5 has good cabin heat when stopped also warms faster. I noticed some strange noise under wife's hood tracked to the aux pump that works only sometimes and vibrates bad when it does work.

The way to determine if your aux pump isn't working is to turn the ignition on without starting the engine. Turn the heat up to max, then take the cap off the coolant tank. You should see a small stream off coolant flowing into the tank up by the opening. If you don't, your aux pump is toast.

yup that's the best way for sure, but if it's weak and you don't know how strong the 'pee' stream is normally it's difficult to access if it's working poorly.

I have 2 3.0L Xers and the temp flux you describe is exactly what I see on the unlocked OBC temp on both. On the highway at a steady 80 mph, the temp will go to about 90-92 degrees then periodically trickle down to as low as 76. Then it will work it’s way back up, but usually only to the high 80s then back down to the low 80s. it’s obviously a closed loop feedback system. BUT The temp never settles at one number. seems to always be moving. suddenly back out to 83, then back to 76, then to 80, steady for a while, then drops again and back up again. personally I don’t understand the logic of a constantly variable temp (outside temp the same, speed the same), but I assume BMW knew what they were doing when they designed the DME software.

As mentioned the variable temp is to improve emissions mostly but some improvement in power as well. Sadly to reduce the mad influx of confused owners if there was a temp gauge they eliminated the function of a gauge and made an idiot light with three values LOOK like a gauge.

:bustingup

Cold - “Normal” - and “Oh Shit”....

It’s on my list of to-dos to reprogram my gauge, will probably sit down and do that one night after I’ve had enough of the valve stem seals for the day...

Unfortunately the middle point allows for way too cold and hot. I drove 1000s of miles without getting to full operating temp when my t-stat failed and wouldn't close completely.