My vote goes to clogged channels - most likely in the heater core itself. If the water pump were failing to such an extent as to get no flow to the heater core at lower RPMs I would expect you to see an elevated engine temperature gauge/indication when idling for more than a few minutes.

Well ...

jvl wrote: 16 Dec 2017, 10:27 Bad head gasket?
But then again, the temp gauge coming up as normal does suggest normal cooling.
Maybe clogged cooling channels or failing/slipping water pump? What about lower speeds at higher revs?

I will have to try the higher revs at a lower speed. Thanks

I have a 2004 1/2 S40. Heater blower work fine but will only blows warm air when you are travailing over 65mph. Temp gauge comes up to temp as normal. As soon as you get below 65mph it starts blowing cold. 65 and above no problem. Any clues?

I have a 2001 S60. When I come to a stop the idle goes down to normal and then surges up and down to 1200 down to 700 rpm. Then it will settle down to normal and then surge again. Any clues???

I have a 2001 S60 with a cruse control issue. Engage the cruse and it will work fine then it will release and you can not reset it. If you turn off the cruse switch and back on still it will not work. Shut off the switch drive for 10 to 15 minutes and try it again and it will work for hours ...