Barracuda Rallye dash Temp and Oil gauges underreporting

I'm having issues with my stock temperature and oil gauges underreporting in my 1968 Barracuda. They appear to report about half of the values that I'd expect. I paid good money to have all of the gauges refurbished and they still do it. Could something outside of the gauges be causing this?

About a year ago I removed the dash and replaced the boards because a pin was broken on the left side. I went ahead and replaced the right side too and the gauges overreported to peg and fried the voltage regulator built into the gas gauge so they all stopped working. I removed the dash again and sent the gauges to a guy recommended here to have them refurbished. After that they worked but they all under report by about half. I then decided to do the gas gauge voltage regulator delete and installed an external regulator, and now JUST the oil and temp gauge underreport. The gas gauge seems to be working fine. Could this be due to an issue outside of the gauges like poor grounding, a problem with the newly installed board, or somehow both sending units coincidentally having issues?