I have some doubt about it... @POR, Timer1 is already off, CCP as well.

What you experiment is indeed an RMW issue. You don't want to write to successive output bit by bit, this will cause some problem one day or another, and faster your OSC is, more chance you have to see it happen.

What you want to do is to write to the WHOLE PORT at once instead of single bits.

Here:
PORTC=7
PORTC=0
Goto Here

Use shadow register, modify it, then dump it to your specific PORT, that's my suggestion.