Problems with timing

I have a 97 Geo Metro 3cyl. I have timed this car many times before, pull out the diagnostic connector insert jumper where it says to and attach inductive timing light to battery and #1 spark plug. This time that I tried timing my car the timing dot jumps all over between 10 btdc and 17btdc (the connector is jumpered in the right spot). My distributor isn't a vaccum advance and it nice and tight on the engine, so I know its not moving. What could be causing this? Could it be a loose Cam gear? Thats the only thing I could think of