2650 raid 5 slow write problem

We have purchased several of the Dell 2650 dual 2.8GHz P4 systems, full specs:

2 x 2.8GHz P4 Xeon

4GB ram

2 x 15GB 10K rpm drives in raid 0 for system

3 x 146GB drives in raid 5 for data connected to Perc3/Di

Linux - several versions tried 8, 9 and WS

Ok, we have a special capture card that captures 150Mb/s of data to the internal raid disk.  This 150Mb/s is about 20MB/s or 20 megabytes per second.  Now, this should be no problem for the raid array or the disk drives.  We capture this data to disk for roughly 14 minutes straight, into one big file.  Now everything works great most of the time, and for most of the capture event.  However, normally every day, we get this strange situation where it takes the raid array like 20 to 25 seconds to write out its buffer.  This causes our system application to die due to the lost data that it cannot capture while it is trying to write out buffers.  I mean, we are not asking for huge feats from this raid array, just 20MB/s, and it cannot even do that.  I have tried both A10 and A17 bios.  I have tried several versions of the perc driver and bios.  Is there something that I am missing?  I hear alot of people say that turning off hyperthreading does wonders, but I don't understand why...