Nobody has an idea I see. =/

Well I've got to admit, it's a bit depressing that my most favorite plant in my collection is withering away slowly and nobody has an idea as to fix it.

I'm sorry for the lack of responses. Some of the sub-forums here are pretty inactive. Can you elaborate on the plant's living situation a little? What kind of soil is it in? Have you changed your watering schedule, or added something to the soil? It's possible it's getting too much water. If you're not doing anything differently, then maybe your soil has compacted over time and is holding more water than it used to. (This happens with bagged potting soil.)

If this type of tree normally would get full sun "in the wild", maybe the window isn't providing enough light for it, now that it's larger than it was. (You might notice this when taking photos-- without flash the indoor photos seem dark compared with outdoor ones. It seems dark to the plants inside too!) Not sure, but do the yellowing areas line up with the bottom edge of your blinds?

Also, how long has the bonsai been in the same soil, and do you regularly fertilize? The soil might not have enough minerals, or an imbalance of minerals.

From lurking on these forums a while, it seems like the most common problems are a waterlogged soil that never dries completely, and not enough light. Can't help you with the light situation but you might check out the Container Gardens subforum for more info on well-draining soils.

Here is a link that might be useful: Container Gardening