The sounds get changed because of programme changes for each channel in the midi sequence tracks. They work well on a soundcard with a GM spec set of presets, but not so well on the Giga because it simply doesn\'t come with 128 GM sounds on board.
Edit or filter out each track\'s programme changes in your sequencer and you won\'t have the problem.
Alternatively, if you don\'t want to use your own sounds, and want to keep the patch changes which come in the sequence, have a look at the Conexant 500mb GM CD rom. I think it\'s available via Nemesys.