1) use autotuning
2) reduce the duty cycle (#define MAX_BED_POWER 48 // limits duty cycle to bed; 256=full current)  ; for my setting 48 is good
3) REALLY IMPORTANT : use temperature table 1 (#define TEMP_SENSOR_BED 1) ; the default table 6 is completley inconsistent for temperatures around 40°C

At this point the needle hotend is perfect. the heating of the syringe(bed heating system) has still 2-3 °C of oscillation , but I suppose that is due to the structure of the heating jacket for the syringe with the temperature sensor not really close to the thermistor.
Probably I should split the thermistor. So, I do not understand nothing in electronics : the two new thermistor should be in parallel or serial and with  doubled or halved ohm?

You need to autotune the PID controller. Covered many times here.

  In general, this. However, I would say tuning the *hot end* pid has been covered many times, but I don't actually recall the bed PID being covered many times. It's presumably a similar Gcode command though.

The gcode for autotuning the bed PID will give you a P,I, and D value that you then need to change on your firmware, either by uploading the code again with the values edited in the sketch, or through your eeprom if that is available.

After getting the variables I assume you'll find all the values slightly shifted down, and if you're still overshooting, drop the P a bit 10-30%.

Other than that, you just need to wrap your syringe in aluminum foil, then kapton, then nichrome wire, then kapton, then aluminum foil, then any tape.  You probably want the nichrome to be >3.6ohms, since that's the resistance they use for the hot-end (i think? something like that.) and you probably need much less power. Guestimate = 8ohms. That's 1.6 ft of nichrome 29 gauge btw, so 29-30 gauge will probably be good, depending how big your syringe is.  You also will want to turn PWM-bed on in the firmware (its bang on-off by default I think)

http://www.thingiverse.com/thing:26047