Computer starts up normally, a few minute later shutdown with no explanation -8200 Ellite Microtow

My Office computer (Intel Core i5-2400 (3.10 GHz, 6MB cache, 4 cores Ellite 8200 Microtower)of above model of 2013 (purchase) starts up normally , after few minutes (10 - 45 min) computer shutdown while working with no explanation with sometime off-white screen or blue screen for 3-5 sec and than computer shutdown totally. what is the problem I can not understand please help me. same model 4 computer has same problem. all thing done from thermal paste , SMPS change etc nothing happen.