Out of alignment. Have the alignment checked. Could be a broken tire or an allignment problem, if its the tire rod your tire will move easily when jacked up and off the ground this is bad and could result in your tire falling off... ouch WRONG WRONG WRONG... ITS BECAUSE YOU CANNOT ADJUST THE CASTER OR CAMBER...YOU NEED TO BUY A KIT...AND THEN GET IT ALIGNED...
The primary cause for jerking on a 99 Mercury Cougar is the engine not getting enough fuel. This can be a sign of a clogged fuel filter, bad fuel pump, or clogged fuel injectors.
you dont need to cause it has a factory tachometer
probably the fuel pump?
I had the same problem in my 1999 Mercury Cougar, I replaced the flasher located in the blinker assembly. It was very simple to do. Flasher cost about $10
The most likely cause is a dirty fuel filter that starves the engine.
my son bought a 1996 cougar that felt like the transmition was slipping and we had the transmission flushed and it fixed it.
the ignition timing set wronge. BAd accelerator pump in the carburetor
it is in the manual says when this happens it is 02 sensor and if it is flashing you have a miss in your engine which can cause damage.
The gears in the speedometer are stripped. If the odometer is working then you have input.
check the recalls for the 99 mercury cougar!! it includes a recall on the throttle cable...
* No power steering fluid in reservoir * The power steering pump is not operating
Your intake manifold probably has a crack in the water jacket.