Jitter and Payload calculation for a CPIPE service 7705 SAR

[weby]

Hello,
We use a Cpipe service between one SAR 8 & one SAR 18 for interconnect 2 differents PBX via an E1 configured in unstructured mode. The connection working, but on the interface of the SAR 8, there are regularly Underrun and Overdun errors appears.
The Cpipe service was create with default Payload and Jitter values (packet jitter-buffer 5 payload-size 256). I have tried to modify theses values but I couldn’t find the good compromise between the Jitter and payload value
Someone, can help me to understand the relation between Jitter and Payload and explain me how I can calculate the good value.
In advance many thanks,
Michaël