[Elecraft] Trouble with K2 PLL reference oscillator

[Luc Favre]

Hello YLs and OMs,
I have an annoying trouble with the PLL reference oscillator in my K2
(1028):
I use the K2, ever for hours, without any trouble. I switch it off.
I switch it on within minutes : the PLL reference oscillator does'nt
oscillate anymore
I switch the K2 off and wait for 10 to 20 minutes : the PLL reference
oscillator starts then like a charm.
Of course, I've inspected and redone all the solder joints, looked for
solder bridges, defect or unsoldered parts, and even more. I've tested  the
oscillator transistor Q19 and the diodes D18 and D16, looked for defect
resistors or chokes (L31) and other discontinuities without detecting
anything false or suspect.
I tried to reproduce the trouble by heating the board with an hair dryer,
without any success.
Has anyone verified such a annoying behaviour ?
Any idea ?
Thanks for help and suggestion.
73
Luc F6HJO/HB9ABB