Sorry, you're wrong.
1366x768 existed, yes... In very select models and up to 2008.
Here's the 2006 line-up:
50PX60 and
50PX600 were 1366x768 - nothing else was. Next year (2007), 1024x720 was out and 1366x768 remained in
50PX75 and
50PX77. Always on the 50 inchers, only. In the prior production years to 2006 they also existed in the 65" models (like with 65PX500).
In 2008 there were only two plasmas with less than 1080p, 42PX80 (1024x768) and... you guessed it,
50PX80 with 1366x768. Some markets still retained a 852x480 model as a entry product after 2006, I think, but not for long (2008 at most, and I think it was 2007).
And from 2008 year onwards that ended. There was just 1024x768 in X10, X20, X30, X50, XT50 and X60 (in both 42" and 50" sizes) and 1920x1080p on U, S, G, V, and Z series with any inch size going on.
In reality, very few Panasonic plasmas were 1366x768 and that size was never on the most sold models (entry ones at the entry size available), if anyone has a 720p Panasonic or Samsung plasma at home there's at least a 95% chance it's 1024x768.
Even if it was 1366x768 that wouldn't change the problem with vertical 720p scaling for scanlines might I add since both solutions are 768 pixels tall, it doesn't look horrible, it just doesn't look accurate for a scanline pattern if you're looking out for it.
--- I have one.Master system would help, thing is Mega Drive 1 has internal interference going on, which I think we spoke about a few pages back.
I wonder if nuking the composite video line on the VDP (as a lot of people do lately) could improve it. Seems like composite interference to me.
EDIT: I confirmed and the cable you bought seems to not be a composite sync one but composite video as sync so you can't nuke the composite video pin inside(you can contact and ask though, just to be sure). Anyway that seller usually refers to such cables asas "sync-on-luma" or sync boosted or raw sync. As seen
here (don't buy that one! it's JP21, it's just because it's the other currently listed genesis 1 cable and higher end).
can you also take a picture at Sonic the hedgehog tile screen? To look for jailbars which would be consistent with interference.