Digital thermometer modification: halfway through

Expect loads of pictures!

After I got that idea about modifying a digital thermometer I couldn't stop thinking about it. So I decided to do it right away. First I had to disassemble the thermometer. The procedure is simple and easy, only tool needed is a small flat screwdriver to press some pins and the case opens. The insides can be seen in the picture below.

1 - two LR44 button batteries; 2 - LCD screen; 3 - case, upper half; 4 - plastic spacer between the LCD screen and the circuit board;
5 - circuit board; 6 - case, bottom half

Next I had to find the correct pair of connections that I could use to determine the temperature. After some testing I found out that I have to connect the + source to the connection marked on the picture below.

Once I soldered all the wires and a female 3.5 jack connector I reassembled everything and the physical work was done. Results:

Multimeter reading is in Volts.
After some experimenting I found out that when temperature increases, voltage drops.

As you can see on the multimeter, I can measure the voltage without any problems. The maximum voltage would be 1.50 V so there is no harm to the computer.

The problem occours when I actually plug the jack into the computer Line-In. Suddenly the voltage drops to about 5 mV and there is no way to increase it back to the previous normal level. Voltage measurement of the computer's Line-In input:

No explanation for this yet

If you have any suggestions what might be causing the voltage drop and how to fix it, please let me know.