[a2-16v-list] Shifting backfire

[John Lagnese]

Hi,
I'm getting a curious problem in my daily driver. Its a 91 Jetta GL 8V. Its got
194K. When I shift above 4k there a small pop, like a backfire? Do I have a
sticking valve? Its got a new mid and rear muffler, so the exhaust is tight.
Also same vehcile, why would the timing mark jump around? Is my timing belt
tired?
Thanks,
John