Seond hand i5-6600k is it broken or could the memory be my issue

Just purchased a second hand i5-6600k from a well-known second hand shop in the UK and I suspect it might be broken. Basically it refuses to turbo boast for long periods at standard clock speeds. Point to note is the processor is the only second hand part. Big however if I underclock the memory to 2133Mhz then this does make it more stable but the memory is rated at 3200 so should be good for that???


Spec of PC.

• 
  i5-6600k
  
  Corsair Hydro Series H45 Rad or Raijintek Aidos CPU Cooler, I tried both
  
  AsRock Z170M-ITX/ac Skylake ATX Motherboard (Bios updated to latest).
  
  Palit GTX970 tested in a friend PC seems ok.
  
  G.SKILL TridentZ Series F4-3200C16D-16GTZB 16GB (8GBx2) DDR4 3200MHz C16 1.35V Memory Kit
  
  Arctic Silver 5 Thermal
  
  Corsair VS Series VS450 ATX/EPS 80 PLUS 450 W (yes I know this PSU is on the very limits)
  
  Thermaltake Core V1 Mini ITX Cube Case
  
  Vertex SSD 256gb (taken from my old laptop)
  
  No CD/DVD or 3.5 inch HD
  
  Formatted windows with no extra programs installed
  
  Reseated the memory

Problem

When I used prime95 28.9 after about 15 cycles of the test the turbo boast fails and drops back to 3500Mhz and overclocking causes an almost instant failure of Prime95 of core 1, it errors with worker stopped. Big However if I underclock the memory to 2133Mhz then this does make it more stable however the memory should be good at 3200Mhz all day long should it not?


Temps

CPU and 120mm Case fan at 100%

Idle temp around 25c in windows

At stock speed 3500mhz/3900mhz the prime temps are

• 
  current temp Max
  
  core 0 63c 73c
  
  core 1 60c 65c
  
  core 2 60c 64c
  
  core 0 59c 62c

Troubleshooting steps

reseated CPU, checked that the protective sticky label has been removed from the bottom of the cooler.

tried GTX970 in another PC and appears ok other PC was AMD FM2+ so that is the only part I could swap over.

defaulted BIOS

tried in bult OC setting within motherboard

changed memory to XMP profile and lower memory to 2133Mhz however the memory is rated at 3200Mhz so should be good at this speed.

changed cooler from Raijintek Aidos cooler to Corsair Hydro Series H45 Rad

manualy set Vcore in small 10mv steps then tested from v1.2 upto v1.4 but no joy.

formatted PC, no extra programs.

any overclock of 4100mhz or more is basically an instant fail in prime95 with core 1 failing almost straight away, I should be hitting a minimum of at least 4200Mhz with this setup and realistically 4500mhz with the lowend AIO Rad.

reated the memory sticks.



Question

Do we think it is the memory or the processor at fault? Or am I missing something obvious? Currently doing a memtest? Is the standard a full 24 hours in memtest86. If doing a memtest then Im guessing I need to do this at both speeds 2133mhz and 3200mhz.



In advance thanks guys.