The temperature of my thermocouple seems to be backwards! If I heat up the probe the reported temperature goes down

This is likely caused by the thermocouple wires being labled incorrectly. Try swapping the two thermocouple leads, even if yellow and red wires are in the right slots - we've seen some thermocouple where the wire colors are wrong.

My MAX31855 output is really unstable and noisy - If I touch or move the probe the temperature output goes crazy

The MAX31855 is surprisingly sensitive, we've found a good way to fix this is to place a 0.01uF to 0.1uF capacitor across the thermocouple leads (that is, place the capacitor into the blue terminal block, or solder to the bottom as shown below).

My thermocouple output has an offset / I have multiple thermocouples and they dont 'agree' on a temperature

K thermocouples are not precision temperature measurement devices! There will be offsets & differences between thermocouples. Most thermocouple thermometers have the offset corrected in software which is what we suggest.  See this guide for tips on calibration:  

Sensor Calibration

For precision temperature measurement, we suggest a 1% Thermistor.

How can I connect multiple thermocouples?

You can connect as many MAX31855's as you have pins. Simply share the CLK and DO pins of all the breakouts and have a unique CS pin for each one
Then you can create new thermocouples using the following style:
Adafruit_MAX31855 thermocouple1(thermoCLK, thermoCS1, thermoDO);
Adafruit_MAX31855 thermocouple2(thermoCLK, thermoCS2, thermoDO);
Adafruit_MAX31855 thermocouple3(thermoCLK, thermoCS3, thermoDO);
You can also try having same CS and CLK pins but all different DO pins
Adafruit_MAX31855 thermocouple1(thermoCLK, thermoCS, thermoDO1);
Adafruit_MAX31855 thermocouple2(thermoCLK, thermoCS, thermoDO2);
Adafruit_MAX31855 thermocouple3(thermoCLK, thermoCS, thermoDO3);

At extremely high or low temperatures, the measurements are not correct

The 31855 chip handles the linear range of the K-type thermocouples very well.  It does not provide correction for the non-linearities that occur at the extremes of the measurement range.  Thermocouple linearization for temperature extremes requires some curve fitting.  See this guide for more information and example code:

Thermocouple Linearization

Last updated on 2015-05-19 at 07.29.43 PM Published on 2012-07-29 at 11.58.38 AM