I must have accidently touched the shortcut for stretch because one of the five tracks was twice the length, reaching far out to the right. CTRL+Z did not undo it.
If you know a place in the manual, or a video that explains this, please let me know.
You can reset the speed of a part to 1x in the part speed setting in the track/part properties box (right click on the part and select Properties from the popup menu).
Flavio.