I wouldn't retire it unless it is hopelessly broken. The money saved by using compatible cartridges in the iP5000 versus a new "chipped" inkjet can be well over $1,000 per year depending on printing load of the user. Even if it costs $300 for Canon to repair it that cost could be re-cupped in a few months.
To the OP, it might be a print head gone bad. A print head runs $50 online so it might be worth the risk to try. Also, you might want to call Canon and get a price for repair. They will typically ship you a refurbished printer very quickly and you return your printer in the box you received the new one. Canon refurbished my N2000 for $300 and that was a great deal considering the print head alone on the N2000 is a $500-$600 item.
The iP5000 is a fantastic printer and one of the best non-chipped printers Canon ever produced. It is economical to operate and and well worth saving, if possible.