I recently had the same problem after swapping to a new rig (IC-9700). After some time of just monitoring, the waterfall would freeze and nothing decoded but the clock still running.
Needed to close WSJT-X and restart but it would eventually happen again.
The reason was because my PC has 6 USB sockets on the back with 12 USB cables requiring connection, I use 2x 4-way USB hubs.
The cable to the rig was connected through one of these hubs (it isn't a problem with my other TRX's).
I swapped the connection to a direct socket on the back of the PC and since then I can run WSJT-X with no hang-ups.
Solution to the problem found by chance.