Why are you not using digital pins?
19 was what I had left! I can try one of the standard digital pins if you think it matters. 18 was floating by the way which I suppose might matter.
You want software serial to control the pins, right? Don't go mucking with the pin mode, then.
I've seen examples with and without the pinModes and I've tried it with and without - and with and without external pullups.
Is your breadboarded 168 using a 16MHz crystal/resonator?
No! that's why sloppy timing is a suspect. I'm pulling the hardware apart now to add a crystal controlled board. I'll just be kicking myself if the problem is still there.