While typing a key will randomly not register and require several attempts at retyping it. In addition, if I'm playing a game that requires holding a key, it'll switch between either refusing to repeat at all or stop after a couple seconds.
Looking at xev, it seems like holding a key down will randomly generate this sequence of events:
FocusOut event, serial 33, synthetic NO, window 0x2c00001, mode NotifyGrab, detail NotifyAncestor FocusOut event, serial 33, synthetic NO, window 0x2c00001, mode NotifyUngrab, detail NotifyPointer FocusIn event, serial 33, synthetic NO, window 0x2c00001, mode NotifyUngrab, detail NotifyAncestor KeymapNotify event, serial 33, synthetic NO, window 0x0, keys: 0 0 0 0 4294967168 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
The KeymapNotify at the end seems to be 2 followed by all 0s usually, or that large number (-128?) in a random spot. After that event, no more keypress events are generated until a release.
This comes after a similar but worse problem I had for a while wherein this sequence of events would occur at regular intervals without user input (confirmed via xev). This didn't affect me too much, infrequent key drops and my terminal's titlebar flashing at random, but was unusable when it came to gaming. In desperation I uninstalled everything related to X and reinstalled it, which traded that problem for this.