How much is your undervolt? When you begin to experience lags that means the voltage is not enough and you have to increase the voltage a bit(reduce undervolting).
Don't undervolt the Intel graphics just cache and core, and make sure cache and core voltages are the same.It is possible that you might need less undervolt. I suggest you to run aida64 stress test to check the system will crash on -80mV.
Go to tools tab and there should be an option to open stabilty/stress test and a new windows will appear.
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