I have a poulan 42'' cut riding mower that revs up good but as soon as i put the blades down to cut the engine almost drops to an idle & the mower will barely move. Lift up the blade engin

SOURCE: craftsman 42" riding lawn mower blade free spinning but wont cut

It sounds to me that you need a new mower deck drive belt, or your blades need sharpening. If the blade edges are rounded, they won't cut butter... Also, when mower is off, check mower deck drive pulleys to make sure you don't have any that are locked up or are dragging. You can check this by pulling the belt away from pulley and spinning the pulley by hand. MOWER MUST BE OFF before attempting any of these checks. You would be surprised how many people would try this with the mower running.
Hope this helps

SOURCE: mower barely moves forward or

Well you have a fault with the belt which goes from the engine to the transmission being too loose or the transmission itself has failed. Get underneath it and, with the engine cold and not running, grab hold of the belt and see if there is any slack in it. This would be the topmost belt, it should not have any slack in it unless the clutch pedal is pressed down. Don't try this with the engine running unless you have an extra set of fingers and a surgeon at hand. If the belt is good then your transmission has failed and you need another mower as the cost of the transmission will exceed the value of the mower. There are mower boneyards around and used trannys for sale on ebay, but most likely you will be looking for another mower.