Lochinvar Noble Combi nkc-110 cold sandwich problems

Hi all, Just got brand new Lochinvar Noble Combi nkc-110 installed and having cold water sandwich issue when running ANY hot water, shower or faucet.The hot water comes in waves and if I look at the unit when the water are running I can see it heat up to around 130 and going down to around 97 and over again. My DHW temp is set up 120. and I have the pre heat enabled and DHW RESPOND TIME set for 2:00 . called tech support the told me to try and replace shower head to lower gpm but this didn't pull the trick as the unit does not provide a constant temp flow of hot water . Any ideas?? Thank you very much