Brakes 'slipping' - any ideas?

So decided to try to strip and clean front brakes today (my first attempt at this).

All went ok, or so I thought. Re- assembled and went for a test drive. On hard breaking the brakes are what I can only describe as slipping. Plenty of bite and then as if the pads lose adhesion, slip a bit, and then grip again.

Only used brake cleaner. No copper slip accidentally on any braking surfaces.

What have I done?
 
Calliper bolts tight? If it were me I'd just whack the wheels off, check for any errant copper slip and also give everything a quick nip up to make sure they're tight. Are your pads in and seated correctly?