STM32F303CBT6 - Board only visible to IDE when probing SWCLK to GND with multimeter

[Blitz942]

Interesting issue I ran into with my STM32F303CBT6. 

Using an STLINK to try and connect to it and wasn't coming right, probed the voltage on PA14 out of desperation and it worked. Intermittently but it worked

Started testing if it was perhaps acting as a pull down and soldered in some resistors to ground to test. I test 100k, 820k, 1M and 10M. None of which worked

I then tested capacitors 3pF up too 22pF and tried combining them with the above resistors. All of which didn't help

I then added 100nF caps between all Vdd and Vss connections as close as I could get to the MCU, as well as between NRST and GND. Again none of which worked. 

(my boot0 is grounded through a 10k too) 

Any ideas of what could possibly have gone wrong here? Such a strange thing to me, that my multimeter was making it work Intermittently. 

[AScha.3]

Show your actual real circuit. Then we can talk about....otherwise i will have to look for my big blue glass ball.

+

When "suddenly" it connects, with DMM connected, its a sign of bad/missing ground connection or super long wires , both no good idea for a high speed connection.

btw

I soldered a F303 to a "blue pill" board, just for fun.

No problem, works fine.

Never had a problem with these chips.