Sounds like an impedance mismatch (bulbs) or a flasher/relay problem. This is especially true if there are LED replacement lights in place of the incandescent bulbs. Also, a bulb can read OK or look OK - but one of the filaments could be on the brink of going out. I would also check that there is not a connection issue (bad connection in the bulb socket itself). Lastly, check the flasher relay - could be a faultly flasher or a bad relays somewhere that is overheating from the additional power draw from the extra lights.
Double check the fuses, relays under the hood and by the kick panel - then try swapping bulbs out and see what happens. If you remove a bulb and the problem goes away - you found the faulty bulb or socket.
2002 Corolla S, Silverstream, 1.8L 1ZZ-FE (VVT-i)
2003 Matrix XRS, Cosmic Blue, 1.8L 2ZZ-GE (VVTL-i) - RIP
2009 Matrix XRS, Blue Streak, 2.4L 2AZ-FE (VVT-i)
