Perhaps a water leak into the cylinder? You run, you've got hot water (or steam) that leaks into a cylinder, raising compression. After awhile, the excess pressure or water leaks past the rings, and everything is back to normal.
Check the oil for color, pull the plugs after a short run to see if you see water droplets.
Also, if it happens everytime, you can eliminate the water and fuel vapor possibilities by pulling the plugs, turning the engine over for half a second, then reinstalling the plugs and see how it turns over. That will vent any excess fuel or water.
1998 Maristar 200VRS