PLL startup problem

[s_]

I'm using STM32F105RBT, and I'm finding that, in certain circumstances, the phase locked loop is failing to become ready.

1) Boot (sysclk = HSI @8MHz)

2) Set PLL with source=HSI(div2) and multiplier=9

3) Enable PLL

4) Wait for PLL ready (waits forever here)

Although the above works ok from a cold boot, it fails after resetting (using NVIC_SystemReset())

Any ideas ?

[Tesla DeLorean]

Doing anything externally with NRST pin?

CPU doesn't run pseudo code.​

[s_]

There's not much attached to NRST except a 100nF decoupler.