Definitely the source - I'm familiar with the widescreen switching issues. Unlike some modern plasmas, the old Pioneer auto changes according to the source OK.
The BBC often puts it's own black bars on when it has to show 4:3 material from other sources and remains in widescreen mode, though it's own old programs are normally shown in 4:3, letting the user decide what mode to use - Just, Wide or 4:3 etc.
Another well encoded source is the C4 cricket, though I suppose the speed of action is a bit slower
