13 PSI @ 38deg F will overcarbonate Newcastle, as an English brown ale, I would say no more than 2.2 volumes of CO2 for that beer, which puts your desired pressure at about 7 psig.
Your altitude puts you about 1.03 psi lower than sea level, that and adjusted for rise of approximately 4 feet from the center of the keg to the faucet, you should push at about 9 psi or so. Keeping the Balance is key! About 4 to 5 feet of 3/16" ID tubing is perfect restriction for these settings.
Try pushing at the lower pressure and see how you do, if you've been driving the beer at 13 PSI for long, you might have as much as 2.6 volumes of CO2 in solution, so you may need to agitate the keg and vent to drive some of that off.
Good luck!
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