IF power is already on.
When you take MCLR low, does it reset and restart everytime?

If the power supply rises very slowly when turned on, the PIC may not RESET.
The minmum is 0.05 V/ms, so it must reach 4V in 80ms or less.

Really BIG caps can slow it down. But if it worked on the 876 it should still work on the 877
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Since you moved the oscillator, did the CAPs get in the right place too. Crystals won't always start up if 1 cap is missing, but still may sometimes.
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Grasping at straws.

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