I have problem with my travel website in yahoo. It was top in almost every competitive keyword in yahoo. To make it more stable position I have submitted in yahoo directory and I got listed there within a week but after 2 week I fell down from yahoo search results with every keyword. It is now more than 1 year but no luck.
similar here.... Yahoo is trying its hardest to act as tough as google by penilizing sites. I know of a site that was ranking high for main terms and then got the same punishment - thing is ranking high in Yahoo doesn't really count for much anyway, concentrate on Google - it's where the moneys at!!
Just trying to improve its value, try submitting to high PR sites instead of the Yahoo directory. Also submitting to DMOZ might work. There are only a few sites to which submission is really helpful so you should try and submit to them.