when it's doing this the upper rad. hose hot andtight, and the lower one is cold?

I HAVE A 2006 BMW325I after it sits over night you get in a drive for about 10 miles then the temp light comes on yellow then red and the vents are blowing cold. i stop for five minutes and the start up & go a bit and then the light will either blink yellow and go off or go back to red for 5 minutes, the fans come on high then l ight goes off vents are warm... when it's doing this the upper rad. hose hot andtight, and the lower one is cold, i've replaced the thermosat assm.. still the same problem... perhaps plugged raditor if hwo do i check it ?? thanx

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[Kerry at Pelican Parts]

Plugged radiator or bad water pump, no circulation

- Kerry