In Australia, Choice (our version of consumer reports) refers to the that buildup as Scrud.
Fill the machine with hot water and 2 or 3 cups of dishwasher powder.
Let it agitate and then turn it off for the night and let it soak.
Let it complete the cycle, then put some old towels in, a good quality detergent and run it through another long hot wash.
You should find the towels covered in bits of gunk, this'll be all the stuff that you've flushed out of the machine.
From then onwards, use the hottest water for a given load that you can, and the issue shouldnt happen again.