Are you sure this was clock drift? My reading of it was that the precision of the clock deteriorated, not the accuracy.
They used a limited number of bits to store an unlimited integer, so the precision got worse as the least significant digits were rounded.
A crude example, if you only have 4 digits to store uptime in seconds, as the time progresses:
0.000 to 9.999 s -- precise to 1 millisecond, 0.5 ms rounding error
100.0 to 999.9 s -- precise to 100 milliseconds, 50 ms rounding error
1000. to 9999. s -- precise to 1000 milliseconds, 500 ms rounding error
Presumably, at some point the error in distance: (time rounding error error) X (target speed) would place the target outside the targeting envelope of the radar, and hence the target disappears.