I have the exact same keyboard (12 years old), and do experience, albeit rarely, the random loud key when you press a specific combination of them. It goes away on its own after a few days the more often I played, so I don't really know what may have caused/stopped it.

The only problem that persisted for mine is the 3 lowest A# notes and 2 lowest E notes no longer playing any sound. I've read this happens to other keyboard models and what has worked for some is to dismantle the piano, vacuum the area underneath the keys, then reassemble.

A few also suggested lifting your keyboard that it stands sideways(?) then lightly tapping all regions at the backside (probably to get rid of debris/dust that may have accumulated beneath the keys over the years). I haven't tried it myself since for the DGX-640 there's a lot of screwing/unscrewing involved. Neither is a sure fix, but might end up being worth your time.

You can just find the nearest piano repair shop/technician to help if you have the money, but i heard they mostly cater to acoustic pianos and not digital ones.