I purchased  a copy of Captain Sim B767 last week. It loads OK and most things work as expected. However, the motion is very jerky and P3D crashes frequently.
P3D works perfectly with all other aircraft on my system including the B777.
Can anybody help point me in the right direction to solve the issue?

If this is the 64bit 767 Captain II (P3Dv4/P3Dv5), then see this topic:
https://www.captainsim.org/forum/csf.pl?num=1626849797

The above Topic is on Page 7 of the 767 Captain II - General forum, there could be more topics on CTD problems with the 767 Captain II beyond page 7, but that's where I stopped after finding the above Topic.

If this is a problem with the 32bit 767 Captain in P3Dv3, then you need to understand that it wasn't supported in P3Dv3, and I don't know of a solution (I installed in FSX, and copied all the files over to P3Dv3 and it worked fine for me).