Long measure reverted on reload

Maybe I'm trying to do something that MuseScore wasn't designed to do, but taking a look at the attached file (Second Air Varie) may give you some idea what happened. I'm going along entering a piece in 4/4 time, then I get to the cadenza.... I change the measure to have 14 1/4 notes in it and write out the cadenza, then put in a bunch of tempo changes (those are the invisible "a"s) to make it play correctly. I was quite happy with it and went on with the rest of the piece for a few measures. then I think somehow--I don't know how, but I certainly didn't change it manually--my cadenza measure (measure 34) half changed back to having only 4 1/4 notes. Now there's a bar line through my cadenza, and when it reaches this point on playback, it plays the part after the bar line at the same time as the next measure. It's very frustrating because I don't know exactly what I did to cause this, and I'm not even really sure what happened.
I'll also include the First Air Varie which has a similar form. I had the same problem when I created this file, but I got it to work by deleting the measure with the cadenza, inserting a new measure and doing it all over again--a lot of work! There are still some oddities with it though: after the cadenza (measure 32) there is a 1/8 courtesy time signature for the pickup measure of then next movement. I don't want it but I can't get rid of it. Also, I tried extending the break between variations by adding beats a measure like 57. I think musescore must not like it when a measure has more beats that it is supposed to. because I get the same sort of weirdness when I do that.
Has anyone else come across this? or have any ideas? I'm using rev 1598 on Windows XP. (haven't gone to the latest rev yet because of issues with repeats)