When switching from the PM assembler to MPASM, and not including config settings in your
code, make sure your config settings in 16f876A.INC are the same for both assemblers.

For a 12MHz oscillator both should use HS_OSC. XT_OSC may not be providing enough drive
for your 12MHz crystal or resonator, resulting in some odd harmonic of the fundamental
crystal frequency.

That would for sure cause garbage to be displayed in a serial terminal. The actual PBP
library code generated at compile time should be pretty much the same for either
assembler. Both assemble the same library code.