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If it improves once you turn it off for a bit it sounds like you are on the right track, Timing again I just changed mine on the weekend as it had very poor airflow all the time, on mine '92 if you remove the glovebox and look straight in there is a plug going into a piece of plastic on the evap case (this is fan speed resistor) if you unscrew it 2screws and look in with a torch you can determine how blocked up it is before you even add the ice possibility. mine has an electronic t/stat as opposed to the old school capilary electric type and i doubt there would be any audible indication in the cab if it is functioning. generaly speaking in Queensland humidity a blocked (with dust /lint)evap will be more likely to freeze up even with a good t/stat. You also would not know how well the t/stat is positioned untill removal as it is on the sid you cant see in situ, some people are a bit rough when replacing evaporaters so if it has ever been done before it may be dislodged. mine was choca block with lint I slipped a new one in and now she blows!
Cheers Andrew