TCX upload failing into GarminConnect

When I tried to import a couple of years of history into GC, it choked on a lot of activities for various reasons. I found that GC was pretty picky about the tcx file it would allow. I was going from Ascent to GC, and one problem I had was on some of my indoor roller rides, somehow the altitude got set to a very large number, which GC would reject. I manually edited those in a text editor using search-and-replace. (BTW, importing these files into GTC would often cause a completely fatal crash that could only be recovered by resorting to a backup file of the GTC database.) I wound up importing the data in smaller bites (months or weeks) to figure out which activities were causing the problem, and then I'd find some way to fix those particular activities.