This article is from the comp.sys.acorn FAQ, by Paul Vigay with numerous contributions by others.
Such freezing during Filer redraws can be caused by applications using 256 colours sprites without palette (it's a bug in RISC OS 3.7). IIRC only sprites private to the task trigger the problem, not the icons used by the Wimp (those in files !Sprites and !Sprites22). Note which applications are loaded when the freezing occurs and add a palette to any 256 colours sprite they use.
 
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