Jasen Betts wrote in a message to Matt Mc_Carthy:

 JB> whaty do you reckon would cause one to loose power ... it takes
 JB> about three minutes to boil a glass of water that seems a little
 JB> longer than it should be... do magnetrons fade? or could it be the
 JB> capacitor?

Magnetrons do fade,  but not very much,  and it takes a really long time. 
You can check connections,  etc. in the transformer secondary and see if
anything looks corroded (I've heard of the filament connections being a
problem in this regard) but make sure that big cap is discharged first! 
Other than that,  check the "window" where the waveguide connects
to the cooking chamber,  and see if anything's gotten into the waveguide
itself,  that could account for some absorption of power.

---