97-Mirage, 1.5 L Timing issues

I have a 97 mirage , 4 cyl, 1.5 L . Had valves ground, new timing belt, head gasket, etc ~9 mo's ago. Had some issues with reusing head bolts ~2 mo's ago. Replaced gasket & head bolts - appears to be holding (180 to 200 psi all 4 cylinders). It ran fine for about a month - but now the car has become very hard to start and want's to kick back. Once started, it runs ruff and back fires when I let off the gas. The check engines lite came on - local Autozone read the error code - said it was the #2 Oxygen sensor. Any ideas why it ran fine for a a while. Did I get the Timing belt too tight or too loose. Could it have jumped a cog? Is there any adjustment to the timing, like the old distributors? What should the timing be set to at idle? :confused:

12 days later - not one reply or helpful hint. I guess this forum is actually just a front for the "Pay to Help On-line Mechanics". see ya!:disappoin