How do you fix this??

I own a 2005 exmark lazer z 60. I bought this mower brand new and I am the only operator. The mower just turned twelve-hundred hours. So here is the problem. When I engage the collection system the mower really boggs down and the rpms drop to a point of almost stalling but work their way up to about thirty-five hundred rpm. Whenever a go up any incline the mower will loose rpm and stall if I don't turn away and get to a more level plain. This has been going on all this season. Up until this season it has run fine. I have re-built the ultra vac, checked all pulleys they spin freely. The engine has a new short block, carburetor, new heads, new clutch. The hydraulics checked out and are within factory specs. No one can seem to figure out what is causing this problem. I had it checked out at the certification school for kohler engines. They suggested to take to my exmark dealer. They won't be able to look at it until March because they have not seen the machine since I bought it. My local mechanic and the kohler people have never run into this before. Thet are stumped. Any help would be appreciated.

How do you fix this??