Me too. I am having a speedo problem that is driving me a but nuts in my 69 Camaro. I'm running the pro comp utla lite 3 3/8" as well.
I bought all the gauges new in 2004. They have worked well except the fuel gauge that is always slow to register unless you tap on the face or go down the road a while. My 69 Vette with the same gauge does the same thing.
The speedo a few weeks ago got notchy, then dropped. The trip odometer continued to work. I yanked it and mailed it in. They were promt to call me and tell me there was a problem with the gauge and they would replace for free

and even re-set the odometer to match!
Got the new one in...put it in the car, and now I have bigger problems.
The car won't shift with the speedo hooked up...AND it doesn't work - but the trip odometer still runs. I have a LS1 with 4L60E in the car. The gauge is grounding out the speed signal. Now...the original gauge just quit reading, there were no driving issues with the transmission.
Called Autometer up. They told me to try a 1K and a 3.7K resistor in line to see if it woud trip. The good news is the 1K resistor allowed the trans to shift, but no speedo reading. The 3.7 kills it just like nothing is there.
So, I put the Snap On Tech II scanner on. I'm getting a perfect reading from the trans speed sender with the gauge unhooked. It picks right up a 1 mph etc. etc. When I plug in the guage it reads ZERO to the PCM and the trans won't shift right. With the 1K resistor in there...I don't pick up any speed signal to the PCM until about 8 mph.
Anyway...they are telling me to force a ground at the speed signal sensor on the grounded side under the car..then test again.
Oh...they told me the NEW gauges have been internally redesigned to be more friendly to a wider range of OEM applications...but they are having trouble with them on GM. Hmmmm. Me thinks...GM is pretty popular, but we may be losing out to the ricer crowd.
I'm going to do what he says, ground the sender to make sure the PCM isn't having a ground issue and go again. If this doen's work (I know my signal is correct due to the scanner reading it loud and clear with the speedo UNHOOKED - I may have to find an older design speedo used and replace it that way.