Ignition timing issue?

I changed the plugs yesterday and the Van seems to run better, but it seems to be using more fuel. I also was running the A/C as it was hot today.

The Van normally gets about 24 MPG and was getting about 21 today. I had my Code Reader with me, so I plugged it in to see what was going on while I was driving, I was not ready for what I watched, and I am at a loss to explain what was going on. The timing varies about 40 degrees with the cruise control set at 65 MPH, anywhere from about 6-7 to around 42-43 degrees. I slowed to go through a town I was going about 35 MPH, with the RPMs are around 1,400. The timing was advanced to around 42 degrees.

This seems to me that that much lead advance at that slow of an RPM, would actually cost the engine power, and actually put stress on the Crankshaft and other major important engine parts.

Is this wildly varying ignition timing normal, or am I brewing an ECU problem?

Thanks in advance!

Jeffrey