thanx for respond .
Actually i think i sort of found out what is happening.
using 2 receiver of the same kind on the same LNB that has 2 outlet was the problem because both receivers are trying to use the lnb and one jumps the other one off because they carry the same interface .
I replaced one of the receivers with another brand and those 2 work no problem from the same LNB.
what is left to check is to put up this receiver with another dish pointing on another satellite to see if it will work without freezing if it didn't then I was wrong from the beginning.