Three things that effect this are the CPU, RAM, and the speed of your harddrive. If you are just trying to play back 30 individual tracks it could be the harddrive. If you are trying to record 1 or 2 tracks with effects while playing back 30 it could be both the ram and the CPU as well. One way you could try to work around this is to mix down your recorded tracks to a stero track (keeping your individual tracks intact) and see what happens. If this fixes the dropout then I would strongly suggest that the problem was your harddrive. However, I think the first thing, I would do, would be to increase your ram to at least 512 MB, more if you can afford it. You also didn't say what operating system you are running, but I imagine that you probably are maxing out your ram with your OS and Cubase. All three of this items, CPU, RAM, and hardrive affect the audio performance with the harddrive, in the past, being the slowest of the three.
M-Audio did have a program that you could download for free that would test your system and tell you what your max for recording and playback could be using their audio cards, the results would still be pretty accurate even for other sound cards. If you can get a copy of this program that is compatible with your system it would tell you what your current system could handle and eliminate the guess work.
Ozlee