Had the same problem on my truck after starting from cold right from when I bought it.
I checked all the usual stuff, belt not bottoming in any of the pulley vees etc.
I could only get rid of it by running the belt very tight, wasn't happy with this as I didn't think the water pump or alternator bearings would last. I changed to
this twin vee belt setup and that cured the problem straight away. perhaps you could get twin belt pulleys for your straight six if all else fails.
Stuart.
