> I've connected up a PIC 16C74 on a breadboard, and am feeding in a
> 4Mhz signal onto pin 13 (the Clk in pin) from a signal generator.
>
> I'm applying 5v to pins 11 & 32, with 0v on 12 & 31

What about /MCLR?  If you don't pull it up, the chip will stay in reset.
What oscillator mode are you using?

> So I wrote a small program (very small :) to turn port D into an output
> and blat bits out of it, incrementing. .  such as this

You're using the outputs "open-collector"?  It looks like you're writing to
the TRIS registers? [bcf RP0 to avoid that methinks]