I think I have the same problem:
Just installed Prepar3D_v4_Academic_4.2.21.24048,
PMDG 737-800/900NGX for Prepar3D VERSION v1.20.8465
and finally FSUIPC5 version 5.123c (registered) on a new W10 PC
Before installing FSUIPC5 the sim runs perfectly, after installing FSUIPC the flight interrupts for a short moment every few seconds.
FSUIPC log shows every second : " **** SimConnect Data Stalled! Re-connecting now ... **** "
Please advice
Herman