Poorly orchid - oncidium/miltonia alliance

[Suse]

Dear All,

I bought the orchid in question which, I've been told by a kind members of this forum, belongs to the oncidium/miltonia alliance just over one year ago. The plant has been doing very well until recently. It has grown three new, plumb-looking pseudo bulbs since I got it and the new bulbs have developed three flower spikes each! The plant still is in bloom (the blossoms have a lovely clove scent), but I am a bit worried that it is not doing so well any longer.
The bulbs look quite shrivelled now. I thought I have been careful when watering it - waiting until the pot got lighter, but maybe I was to generous with water after all. I have taken it out of the pot today to have a look at the roots and they look brown and are a quite soft to the touch. The leave on the old bulbs are turning yellow (but I think this is normal, isn't it?).
Can anybody please advice me how to rescue this plant? I have been thinking about mounting it to avoid future over watering, but only have a window sill ...
I'd be really grateful for your ideas how I could rescue the plant!!!!