RE: RPMs

I have a john deere 325 18 H.P. Garden Lawn Tractor. In the winter time with the snow blower attachment, the RPMs stay at the max with no problem.
In the spring and summer when the mower deck is on after mowing for 10, 20 or 30 minutes the RPMs just drop to about half on it's own. The throttle lever is at the max and stays in that postion. Sometimes if you sit there for a few minutes, the RPMs come back to Max. Sometimes NOT.
I am going crazy, I don't know what is going on.
Anyone experience the same problem.
I maintain this tractor regularly, and am flabbergasted as to this problem.
Thanks,

Broski............
I have a john deere 325 18 H.P. Garden Lawn Tractor. In the winter time with the snow blower attachment, the RPMs stay at the max with no problem.
In the spring and summer when the mower deck is on after mowing for 10, 20 or 30 minutes the RPMs just drop to about half on it's own. The throttle lever is at the max and stays in that postion. Sometimes if you sit there for a few minutes, the RPMs come back to Max. Sometimes NOT.
I am going crazy, I don't know what is going on.
Anyone experience the same problem.
I maintain this tractor regularly, and am flabbergasted as to this problem.
Thanks,
Broski............