Either the hazard switch which is also the turn signal relay or the multi-function switch (turn signal switch) on the steering column. For my '02 it was both.
The next time it happens, wiggle the multi-function switch and see if it quits; albeit temporarily.
Hazard switch/relay is easy to change. Just pop off the key trim and the centre instrument cluster/radio trim, disconnect the electrical connection, depress the tabs on the switch and remove it.
Make sure you have a new switch on hand before removing the old and be careful prying off the trim around the ignition key cylinder. The trim tabs break easy.