what on earth can I be doing wrong?

[skimask]

Make it even simplier:

trisb = 0
main:
portb.0 = 1
pause 500
portb.0 = 0
goto main

No variables, no nothing to mess with anything...

And strangely, my logic probe records a high on RC6. Which is strange since I don't think I've bothered with Port C in any way

It's floating... All (ok, most) pins are set to input on RESET so they don't load down the external circuitry (well, that's one reason anyways). So, it's probably floating. I'd be willing to bet if you put a 'scope on it, grounded out the pin with a 1k resistor, it would go to 0v, then when the resistor was pulled away, it would rise 'slowly' back up to 5v.

Or have I found a brand new way to be confused?

Yep...you have...
Do a search on config bits and presetting, etc. There's a bunch of threads on presetting config bits and clock settings, etc.

[dmacq]

thanks skimask for offering the simpler code.

I think you're probably right about the floating pin. Although, I connected a 1k resistor to it and gorund and the pin is still high. I don't have a scope to check though.
I have checked the config settings and tried every option but RC. But no luck. Tomorrow I'll rewire it for RC and see if that makes any difference.

And Duncan303, I think you're right too. The oscillator is almost certainly the problem. But as I say I've tried every configuration and nothing works. I have a smaller 16f876 pic that I will try in the morning.

So frustrating. But thank you so much for trying to help

[skimask]

thanks skimask for offering the simpler code.
I think you're probably right about the floating pin. Although, I connected a 1k resistor to it and gorund and the pin is still high. I don't have a scope to check though.
I have checked the config settings and tried every option but RC. But no luck. Tomorrow I'll rewire it for RC and see if that makes any difference.
And Duncan303, I think you're right too. The oscillator is almost certainly the problem. But as I say I've tried every configuration and nothing works. I have a smaller 16f876 pic that I will try in the morning.
So frustrating. But thank you so much for trying to help

Try a different spot in the breadboard, or a new breadboard.
We've had more than a few discussions here in the past year or so about breadboards gone bad (I should make a video...I'd make million$ !!!)... contacts wearing out, dirty contacts, etc.