I hope you didn't delete the whole preferences folder! Just the file:
btw, you need to quit Logic before deleting preferences - and it's not advisable with some interfaces to plug them in and unplug them during a session. Yes, it can be done, but I wouldn't do that with any interface...
I really have no experience with the Tascam US122, but I really feel it could be more to do with the interface than Logic.
Remind me again, are you on the latest version of OSX? 10.5.6? Have you done a "Software Update" and installed the Pro App updates?
The problem with an audio interface is that it's not just hardware. It's software too! The Software part of it is a driver. Most interfaces I've used work well with OSX and have well written drivers, but there are a few that have badly written drivers and therefore don't work well.
Again, I have no idea about your interface.
To test this out, try taking the interface round to a friend with a Mac and Logic and using your interface on their Mac.