Could water temperature fluctuations be caused by new plumbing?
The Most common cause of this when there are renovations is cross connection of the hot and cold lines, this can be caused by 


The shower/bath (or other similar fixtures with a breaching piece) are capped with wall caps (or spindles that don't have a washer in them) and the water can circulate between the hot and cold pipes.
The "stubs" under the sink are connected via a pipe externally till the plumber comes back.
The Plumber has cross connected the pipes in the wall (Damn!)