Matchbox-keyboard is under development and probably should be considered unstable. It is even difficult to download anything from the maintainers page. When I have some free time, I will test another version from git. I built it but unfortunately it requires different layout config files, so it is not a quick replacement. Alternatively there are more virtual keyboard projects, which may be taken under consideration here. But I need time, which I am very short of now.
For the time being you may try to download latest version of the whole package from my webpage. I have rebuilt it with some optimization settings, which probably will not be of any help regarding this issue.
A way to restart keyboard when it dies would be a good temporary solution. I will think about it.