after new fan clutch and thermostat, fanclutch starts roaring within 20min.

I had a similar problem with my '97 V-6. I know you said you had just replaced the fan clutch, however that was the problem with mine. The clutch is supposed to keep the fan from running at the same speed as the engine. Try this: start the truck and let sit for a minute to warm up, then rev the engine a little to see if you have the "roaring". If that is the case then your clutch may still be bad. Make sure that you did not ruin the fan spring on the front of the clutch when you installed it.