Walk behind New JD we85 stalls when engaging blades

Need a little help. Won a mower at a charity auction for my Mother in Law. Don't know where they got it from- no stickers on the unit. Starts terrific, continues to run and wheel drive works fine. Having some problems with the clutch when I engage the mower blades. The engines rpm's drop off and it will stall most of the time when engaging the blades. I adjusted the slack of the clutch spring under the deck and now at least I can feel the positive click of the clutch engaging correctly when i push the lever forward and it stays engaged until I release the safety brake handle (when I can get it going) which is what it is supposed to do. Throttle is 4 position and I am using the 3rd setting so it should be getting plenty of rpm's. However, I'm still thinking it's a throttle or choke problem. Any thoughts or experience before I try calling one of the local JD dealers service department for help? I did try calling the venue where I won the auction to find out where they got the mower donated from- it was donated by a local Foundation with an answering machine. Thought I was doing a good deed for my MIL, but maybe it backfired. Thanks

Hopefully you sent in the warranty card, then you should be able to get this corrected at your local JD shop. If not I'm think you should check the belt, something is tightening up, causing the RPM's to drop. If it is new and you didn't send in the warranty card, I would still explain your problem to your local JD dealer, they may have donated it. Most good dealers are willing to help you out.