It will usually be Penatchloroethane that decomposes into Tetrachloroethene, and Pentachloroethan does break down quite easily at room temperature.

If you only have a few samples to run at a time you can store them in the cooler at 4c and that will retard the breakdown for a few hours while it warms to room temp on the autosampler.
Hmmm, I found the location where pentachloroethane should elute and there is not even a trace in any of the samples. Is it possible it completely converted? I am going to contact the PE provider and ask them if pentachloroethane was present.

I still have the original stock. I'll prep it again and run it just as the autosampler is grabbing the vial.