I own an MTD 2560 zeroturn mower. Whenever I release the brake and the plunger switch releases, the engine seems to lose all power, even when moving the throttle to full the engine rpm's stay constant. It doesn't stall, but when I reengage the brake, the engine idles normally. Could the problem be in the plunger switch?

Sounds more like somethings dragging. That switch should kill it.