Troubleshooting a Transistor

First some background, I am running a bunch of solenoids from multiple digital outs on my arduino. I am using a TIP102G transistor, using a schematic similar to the one found here http://www.arduino.cc/playground/uploads/Learning/solenoid_driver.pdf . The solenoids I am powering require 12 volts and draw around .11 amps. OK, so now for my problem :-X, one of the circuits is only outputting around 6 volts and slowly increases to around 8 or 9 volts after a minute or two. I switched output pins on the arduino and still have the same problem with the same circuit. I am fairly confident that all of my solder connections are solid. Does this sound like a bad transistor? Is there any way to test the transistor without desoldering and using a fresh one (this was my last transistor, so I would need to order a few more). Any insight would be appreciated :D.

I switched output pins on the arduino and still have the same problem with the same circuit.

Are you saying that you see the same voltage independent of if the arduino pin is high or low?

one of the circuits is only outputting around 6 volts

Where are you measuring this? If it is on the collector then you should read 0V when the transistor / solenoid is on and 12V when it is off. I find it hard to think of a transistor fault that would produce a result like this it is more likely to be a result of bad wiring or incorrectly identifying the transistor’s pins.
Try replacing the solenoid with a 1K (or so) resistor and see if that switches the voltages correctly.

Thanks for the quick reply Mike. I measured the 6V between the solenoid power (V+) and the collector. To clarify, I am running 14 separate solenoids off of each of the 14 digital outs from the arduino. So I have one circuit with a transistor for each solenoid. As of now, I have 4 of the 14 wired up and only one is malfunctioning. The other three circuits read 12V between solenoid power and the collector and switch the solenoids just fine.

Initially I thought that I had a faulty connection at the arduino pin... so I switched the arduino digital out that was controlling the faulty circuit, and got the same problem on a new pin. I could not imagine bad wiring only producing half of the voltage, but I checked the continuity of all of my connections, and physically inspected all of the jumper wire solder joints, and everything looks fine to me.

I tried replacing the solenoid with a resistor and I still reads 6.23V and slowly climbs to around 8.1V after approximately 2 min.

I am almost certain that something is wrong with the transistor, I might have to order some new ones and try replacing the questionable one. I was hoping that someone had a trick that would let me test if the transistor was faulty. I'll report my findings next week when I receive the new transistors.

I was hoping that someone had a trick that would let me test if the transistor was faulty

Well I have made one and I am in the process of preparing it for the web.

However, your example is so simple then you can assume it is the transistor that is faulty. Especially considering what you have done.