Timing

Just a quick question re timing and what I am seeing.

When using the timing light, I am noticing that the mark say at iniital will usually be very close to TDC using the dial in advance light I have. But I do see it fluctuate a bit and sometimes I see it move to the advanced side quite a bit. Like as much as 15 degrees. So the engine is warm and idling and I see these bumps in timing. It also seems like I hear the engine lump a bit when this happens. With the cam that is in this 340 I have now set initial to 22 degrees and the advance on the distributor is adjusted to give no more than 6 degrees or 12 on the crank.

So my question is are these jumps in advance normal? Could I have a loose timing chain?