Yes, don't wanna hijack the post...I am using a crystal, and its the first time I've had trouble with the osc on a *>single<* stripboard layout...isolated incident.
Nigel, you have recommended your layouts several times, and I'm sure they're just dandy, but my board is slightly different. The crystal is quite close to the PIC, maximum trace length 7mm....via a jumper wire that is. However, due to the generous size of your layout...your xtal/caps circuit seems well isolated from anything else, so I'm guessing its a 'coupled electric noise' thing, with yours being less suseptable.
It's related to 3v0's problem, because the layout of this board is the same his. I had a GND trace inbetween the two osc pins so that I could use a resonator when needed (removing the caps). I too will probably trash this board, I'm completely confused, short traces, clean connections (no cold solder joints), checked with a DMM, caps...its all good.
3v0, could post a larger pic of your layout, so we can see what traces are near the osc pins? Although many insist 'stripboard is fine' is isn't the most reliable for oscillators, are you using SRBP? This issue is bugging me something chronic, lets hope we can sort it out.
Blueteeth