RH850 Software Trace uses a small FIFO that can easily overflow when too many trace messages are generated. This can cause sampling time to decrease or overflow error messages in the Analyzer Trace window (indicated by brown colored lines).
Open Hardware / CPU Options / SoC and increase the LPD clock until debugging works reliably. If the overflows still happen check the next possible solutions.
This can be achieved in different ways, for example:
Emulation Adapters for some RH850 devices utilize NEXUS or Aurora trace interface that is faster, thus enabling more trace data to be transferred off-chip for the same time frame without overflows.
Check out Knowledge Base article about Buffer (FIFO) overflows to see some generic solutions.