You were right, it was overwriting old forms. It was one of those "Duh" moments, SharePoint views 101.
I forgot to increase the number of items displayed in the default view. It was still set to 100. Since I had the default view grouped by some of the fields, I didn't notice it. Because of the grouping and the item limit it was dropping random forms from the default view, so the ones overwritten weren't always the oldest.
I have now increased the limit to 100,000, should be enough, right? :-)
I did do one thing right, the ID is fetched when the users click on the submit button, not during form open. During the first submit it also sets a field called ticket number and during submissions later on it checks first if ticket number is already populated and if it is, it doesn't make any changes.
Thanks. Manny