I noticed for the past couple weeks, when I come to a light or stop, I'll sit there for a moment, and the idle will be around 1000 rpm. Then it will suddenly drop to 500 which just seems low to me, the truck shakes alot, and then it comes back up after 5 seconds or so. This all happens with no lights, no ac, no defroster on. Truck is stock 99 taco 2.7 5 speed 4X4. Any ideas whats going on or is this normal. The way it shakes doesnt feel normal to me. It doesnt happen every time I stop though.