Oftentimes the bulbs in the four way system are dual filament because they serve multiple purposes. The high speed flashing is almost always an indication that you have at least one filament burnt out on that side of the coach. If you have the high speed flash on both sides, your problem may be the flasher, but my money would still be on a bulb. Last time I saw a flasher fail was about 1973.
John & Diane, fulltiming since '12 02 DS, FL, Cat330, '04 Element NHSO RVM103
John & Diane, Fulltimers. RVM103 NHSO
On the road since June '12 with Lincoln, the guard cat.
2002 Dutch Star 40, Freightliner, Cat 3126, 2004 Element