Your intake manifold probably has a crack in the water jacket.
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.
A blown head gasket.
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...
Hey Cari==Make sure there is good air flow around and through the alternator. Restrictions from heater hoses around alternators can cause them to overheat and fail. GoodluckJoe