since there seems to be a relation to the STDM cable (cleaned, worked some time, failed again), it could be that the frequent plugging-unplugging has scrubbed off the surface on the Scope card, usually some (very thin) gold plating.
The same could apply to the connector (which I suspected in my previous post), but the chance is somewhat lower because they obviously use rather 'hard' material on that side.
Now what's happening ? If the (protective) goldplating is gone, there's a copper based metal contact on the card's side, which you clean with alcolhol and rub away the isolating oxide from the surface and get a connection again.
All is fine... for a short while - then the oxidation process will start again creeping under the connection point.
This builds up a small capacitance and the high frequency signal will get reflected or run mad otherwise.
At least there will some wiered signaling on some lines - you know the result.
You clean with alcohol - another cycle starts...
Since you rely on your cards as the heart of your studio - and if you don't care about warranties, well here's a drastic remedy I would apply - but it's really my very private opinion and not a suggestion at all
I would solder a row of circuit connectors (male/male) right on the STDM connector of the card (they match exactly) and make a custom cable with connectors (female) that look like those of regular harddisks, but 20 instead of 40 pins), which fit much tighter.
Those connectors are available in good quality, opposed to the ones that are on a regular STDM cable, which seem to be totally outdated.
I tried a dozen catalogs without success today.
I wouldn't want to convince you to spoil any of your cards, but if they really show heavy wearout on the connectors, this may be an option to consider - see it as a kind of last resort.
Additionally you may ask a jeweler if goldplating of a circuit board's connctors is possibly without risk of damage.
cheers, Tom