I was shown some time ago that if you do the math of a 4Mhz oscillator, internal for a 12f675, the possible timing errors preclude oscillators that slow. Since you went to an external oscillator, assuming greater than 4Mhz, the problem was reduced.
I have always been told the slowest you can run a PIC transferring serial data is 20Mhz. If you move to that speed I'm thinking your problems would be reduced.
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