OK, today i was driving down the hiway and smelled coolant. Got to work a mile down the road, got out a flashlight and a mirror. Found the rear freeze plug in the head was gone!!! The truck is a 1995 3500dually 5speed. 5000 miles on a brand new motor. The temp never went above 200degrees. It is normally at 170. When i got it back together 5000 miles ago the side freeze plugs in the block kept popping out. I made some billet aluminum ones and have not had a problem in a long time. There is a new thermostat in it too. The pressure is usually about 16lbs on the coolant system. The compression is 475psi in the cylinders. No oil in coolant whatsoever. No coolant in oil whatsoever. I even performed a combustion leak test with chemicals. All OK. Im just confused on what to do next. I really need this truck back on the road. I just cant understand why the plugs would keep coming out?!!