Make sure you have installed the latest firmware (free download from iTunes) for your iPhone / iPod Touch. The latest version fixes a lot of stability problems in all applications. The difference is quite considerable so we strongly recommend doing this.
Other crashes can be due to the memory management of the iPhone. Applications are given a limited amount of shared memory. When this limit is reached the iPhone can kill the running application. This can happen if the iPhone checks for emails and you try to send a picture message in Palringo, for example.
Being a member of very large Palringo groups can also cause Palringo to become unresponsive.
We are working hard to solve all of these problems and hope that you see an improvement with each release.