I guess you weren't totally serious there Aran, but is this because "chat" sounds too trivial or something? I guess "cloncan/cloncian" would be even worse in that sense.
BTW, looking up these words in Gweiadur, I came across:
siarad fel melin bupur - to talk like a pepper mill - explained as: siarad yn ddiddiwedd - to talk endlessly.
(I guess we all know people like that... ).
oh, here's another good one:
siarad siprys - siarad cymysgedd o Gymraeg a Saesneg - to talk a mixture of Welsh and English.