I use a Canon i860 and recently had problems with streaky, patchy colours using compatible cartridges from Choice.
I removed the print head and cleaned it under warm running water. It was surprising how much excess ink came out of each of the 4 nozzles. I dried it, replaced the cartridges, did a print head alignment and all was well again.
It's also wise to print a page per day to help stop the cartridges from clogging/drying up.
I use this click here to give all 4 cartridges a work out.