I have a husqvarna 172 ride-on lawnmower. Running

Before you do anything else check to make sure the blades are engaged on thier seat ( the star) and the blade bolts are tight.
If you have an electric PTO this does not apply to you.
After confirming that your blades are tight check the operation of the PTO cable and the pivot bracket by lowering the deck all the way. Laying on the ground on the discharge side of the mower you will be able to look at the components and see and touch whats not moving freely as you engage and disengage the PTO lever. Look for a stiff PTO cable, stiff pivot bracket, missing return spring, broken spindle brackets and broken deck bracket welds.
Of course you should never work on any mechanical device while it is running.

My idea is not about the belts, but is about the deck. If the tensioning mechanism on the deck is getting stuck in an intermittant position than the belt may be just tight enough to turn blades while slipping through the pulleys, this may cause enough friction to start melting your belts. If this is the case you will have to remove your mower deck and see what is preventing the mechanism from functioning properly.

Could be something jamming a blade or belt. With the engine OFF and the key OUT make sure the blades and belts all move freely by hand. Clear out all grass and c**p from the blade and belt areas. You may have to remove the blade deck to do this - instructions are in your manual or download a manual from the Husqvarna website. Good Luck!

mine has a reverse cut out when blades are running....maybe the switch is bad and thinks mower is in reverse therefore shutting down the mower when the blades are engaged....

Could be a broken belt, loose belt (stretched) or something is under the mower deck jamming the blades. (stick for an example)

Same reason a person slows down when he is riding a bicycle up a hill?
Does the engine nearly die when you engage the blades on the fly?
If so you may have a dead cylinder, my 2148 would mow on the flat pretty well but not an incline with one cylinder. Cause of my dead cylinder was bent pushrods, which is not unusual on V-Twin B&S. You can test this by pulling one spark plug wire off at a time and running it.

the lowerer the deck the more your working the mower. either mow more often or reduce speed

YOU MAY HAVE A BENT BLADE OR EXESSEIVE BUILD UP UNDER THE DECK

This is most likely a belt problem. Check the deck drive belt that runs from the mower deck to the engine it/s come loose and got stuck between the guides.