the first thing to do would be to replace all turn indicator bulbs with new, proper bulbs from a dealer. it sounds foolish to buy bulbs from a dealer, but it can make a difference.
if the problem persists, then there has to be either some kind of resistance issue in the circuit's wiring, or a problem with the control module which controls lighting. if you drive for a long time with a short in a lighting circuit, it can actually lead to the need to replace the control unit
one other thing to try, which is worth a mention, is to unhook the car battery for an hour, this will reset everything and can clear out weird glitches - I would start with this, then replace bulbs, actually.
Sorry if you were hoping for a different answer, please don't shoot the messenger - I can only give the honest answer, which I have. Please remember that a low rating PUNISHES me for my honest and prompt response, and I receive nothing for my time and answer until a positive rating is clicked.Please let me know if you have further questions. Please do not forget to click a positive rating as this is the only way I receive credit for my answer, thank you. Best wishes with your vehicle.