1995 Mitsubishi 3000GT. No Tension on Belt!

1995 Mitsubishi 3000gt. Mileage:100,000. Hi, im having a problem with tension on my timing belt, it all started when my timing belt started burning. I took it to a shop right here 2 blocks away from my house they told me that the waterpump completely frozen, I took a look at it and tried to turn the wheel and it was stuck. So they replaced the timing belt and the water pump but when they put it back together the belt was all loose, they said it would need a new tensioner, so they relaced it and the belt got tight and it worked good but the next morning I checked the belt and it was all loose again! I took it back and they said it was a manufacturers defect so they replaced the tensioner again and it was tight again and it worked good but again after the engine cooled down it got loose again, why does it lose its tension? Is there another tensioner hidden somewhere else? Or is ther some kinda trick to fix this?