1994 3000GT DOHC 3.0 non-turbo

What's up guys, I have a 1994 3000gt DOHC non-turbo with 98,000 miles and recently found that I had a bearing in the # 3 piston spun out due to timing and water pump problems around 80,000 miles. Anyway I had never worked on an engine before but did pretty much a complete overhaul of this engine to specs but however I am done to putting it in timing which seems easy. I got it right about 20 tries but when I start it and then shut it off it keeps losing tension around the water pump I bought a new belt because it sat for about 2 years and I found that the old belt stretched a little but before I bought the new belt I replaced the tensioner adjuster and it was losing enough tension to pull the belt of the cam I then took back the tensioner adjuster and bought a new belt and it does the same but not to the same extent should I have kept the new tensioner adjuster or is it something beyond my knowledge?