The 3.1 is very common for fuel injectors plugging up. Take it to an independent Garage and hvae them professionally clean the fuel injectors. The in tank additive will not solve this.
May be part of the hazard warning module and replaced as a unit
possible it could be the cam shaft position sensor or it could be the control module in the distributor.
Check the ignition control module...notorious for this problem
Ignition module.
No it will not cause misfire.
Have the ignition module and coil packs checked.
did you changed your thermostat? You MUST keep around 195 degree thermostat in the vehicle at ALL TIMES due to the computer system. The ignition module usually gets hot due to corroded electrical connections at the ignition module and or the module is HEAT SENSITIVE and needs to be replaced. (plug wires) Ignition misfire can produce a variety of driveability symptoms including hard starting, rough idle, hesitation....
If a defective ignition module, (or bad plugs/ wires) causes a misfire condition, one or more plugs won't fire. The uneven power pulses cause engine shaking.
Could be a broken timing belt or a bad Engine Control Module.
The transmission is controlled by the engine computer. It can be replaced.
Without an engine size we guess but if you can't see them then they're under the center plastic cover of the engine. Cover will have coils,wire boots, module & plugs under it.
should be on right side, front of engine. at upper radiater hose, if not there check the thermostat housing!! on the 3.1 it is located by the ignition control module.