I have an older Exmark Lazer Z HP that seems to be losing RPMs for no reason. It is from about 2001 - 2002 timeframe. The Engine is a 21hp kawasaki. It will seem to be running just fine, then all of a sudden, the engine will sound like it is bogging down and the blades will start losing a bunch of rpms. This happens whether I'm going thru grass or not. It will sometimes do it whether the blades are engaged or not. After a while of this, it will just pop back to full RPM and seem to run fine for a while, then it will do it again for no reason. Could this be a fuel pump or fuel filter issue? I had it to the shop, but of course it wouldn't do it while they had it. I appreciate any suggestions you can give.