Is it possible for cold-soaked fuel to start depositing wax on the interior of fuel feed lines, in much the same way plaques gradually fur-up your arteries?
Just a thought that occurred - during the cruise phase, a high flow-rate may prevent this from happening. Throttle back for a descent, lower flow rate, less friction, lower velocity=higher effective pressure (iaw Bernouilli). Cold-soaked fuel may start building wax somewhere along the inside of a feed line, hence restriction. After the fact, wax buildup melts, evidence vanishes.
Just throwing a possibility out there - I'm sure somebody much more versed in fluid dynamics than I could critique (read - shoot down!) this theory.