Hot water goes cold then hot

I have a two stage Burnham boiler (160F/180F) with an Everhot indirect (coil based) hot water tank. There are separate circulators for the heat (Bell & Gossett) (fhw) and the hot water (Taco F5). There is a Watts Tempering Valve set at 150F (Factory Setting) it was changed two- three years ago along with the Everhot tank. Tenants are complaining that from time to time, the water fluctuates in temp between hot and cold, when taking a shower.
What could be the problem? Where should I look, any ideas?
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