2005 sticking throttle - not mat or pedal related

My '05 2wd 4cyl 5 spd that I've owned since 2007 has had 2 incidents, a year apart, of the engine rpm not dropping after shifting into neutral after going up a long grade(Grapevine in SoCal). Going up the tranny is in 4th gear, ~3K rpm. Once it happens it's repeatable after climbing the next hill. Stabbing the pedal to bring the rpm over 4K brings the rpm back down to idle. I usually coast in neutral down most downhill grades if possible.
After the first incident and lowering mpg I cleaned the throttle body and MAF which produced the expected beneficial results. The truck has 138K on it and this is the first weird issue to occur. The problem hasn't occurred on other grades(e.g. I-80 in Tahoe). This is not related to physical interference with the pedal and the floor mat. The pedal is in its normal non-depressed position with engine humming along at 3K rpm. I could put my foot behind it and not move the pedal back.

Anyone else experienced this and cured it? And yes, I realize not coasting in neutral down a grade would certainly solve it. Nevertheless something isn't right.