2 different Motif synths have same detune issue

I have a Motif XF8 I got from the Oak Ridge Boys 3 years ago, and I put expansion card in to use the American Grand sounds in it. This has worked beautifully, until very recently. We have noticed the keyboard will start pitching downward while being played. Changing patches or a restart fixes problem, but for only a few minutes. After live chat with Yamaha back in March, I took it to their recommended repair guy here in the Nashville area. He has yet to open the unit up to find if there could even be an issue (dropped off March 15!). Thankfully we have an original Motif 8 (Also from The Oak Ridge Boys) that we have been using while we wait for a repair. Now THAT keyboard is doing the same thing!! We played a show this past Saturday night at The Franklin Theater, and I began doing some meditating on what could possibly be the problem. This is what I have pondered. I have some custom cables made by ProCo for our onstage keyboards. They consist of 2 XLR’s and an extension cord for power (edison plugs on each end). 2 audio lines for patching the synths to the DI and the power cord running back to the stage box and a power box. The cables are well made, however they did use very cheap parts for the AC connectors. I am wondering if the AC might be unreliable thru the cheap plugs enough to cause this de-tuning phenomenon? I am replacing the plugs with heavy duty parts. Maybe a tube amp with voltage dropping down to 90 volts might still operate, but I am betting synths with faulty ac could be unhappy and not play nice. Would love to call the repair guy and say “Never mind opening up the synth, it is fine. But thanks for keeping it for 3 months and not even opening it up to see if in fact there could be an issue under the hood.” So I suppose my question is sketchy power due to the custom cables coiuld be causing the de-tuning issue. I have left the old Motif on 12 hours at a time in my studio this week, while checking constantly with a tuner. It hasn’t detuned one time. I even messed with pitchwheel too, and still back to 440.