If a tachometer is connected to a coil and is reading out the wrong revs and the settings on the tachometer are correct what makes this happen??

Also the car violently holds back like the injectors have cut out at what it says to be 4500 to 5000 rpm which is definately not right and I reckon its actually about 3000 rpm.

If I turn the lights and heater on it will do this at even lower revs

Please help as I have replaced the coil and now don't know what to look at