Hot water system doesnot work properly.only hot or cold water is coming but doesnt come mixer?

we have gas hot water system.but the problem is whenever we used mixer of water,few second hot water is coming and after few second its become cold water and if we needd hot water we have to turn off the tab and restart from the begining.during shower we have to do these things five six times.please provide me... show more we have gas hot water system.but the problem is whenever we used mixer of water,few second hot water is coming and after few second its become cold water and if we needd hot water we have to turn off the tab and restart from the begining.during shower we have to do these things five six times.please provide me proper guidance so i can fix my hot water system.thanks.
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