I have a Sears 42" cut that I have used extensively for 8 years. This year the mower blades slow down when under even a small load. Is this the belt slipping or something else? I have never changed the belt as I have never had a problem before.

When was the last time you sharpened the blade? Also how is the belt tension on your unit?

Just changed the blades this year. Is there a way to adjust tension or is the tension determined by the springs on the pulleys?
Thanks!

On some decks you can add some tension by moving the bearing that the shaft is attatched through. Almost like a cars alternator was in the old days but not very far. Otherwise your springs can caus the tension to occur. You can also try a product like "belt dressing" that usually works pretty good too but be sure to clean the belt so nothing accumulates on it.

Clean off your deck,lube up the part that moves back and forth when you engage it.
Sounds like its not going all the way in .
If you have never change the belt you might look at this also,