putc() timing problem with large char buffers

[cian]

Hi,
I have some code that amongst other things is outputing some AT commands to a Siemens phone via putc().
I'm using a picf16lf876. I have a large character array (45 characters) defined and various other variables and everything works fine, however when I try and define a second array of 40 chars, putc() no longer operates reliably.
Examining the output on a scope shows bit widths larger than they should be which results in extra 0's or 1's being seen when there are 3 or more of them in a row.
I'm nowhere near exausting the RAM (these guys have 368 bytes and I have the 16 bit pointers enabled), memory usage is about 50% at worse case.
Even weirder is that I can increase the size of my original array to 85 chars with no ill effects on putc.
Literally commenting/uncommenting the declaration of this second character array makes putc() work/not work.
Anyone have any idea what's going on, how can an extra character array be affecting the bit timing in putc()?
Cian

[PCM programmer]