305 & 306 are misfire codes for adjacent cylinders #5 and #6. Possible blown had gasket between the two cylinders? P0316 may be set if any of the cylinders are misfiring. P0316 will be set if any misfires occur during the first 1,000 engine rpms (about the 1st minute of operation).
I didn't take the time to see how the cylinders are numbered (1,2,3 left side, 4,5,6 right side) or (1,3,5 left side, 2,4,6 right side).
So, with two cylinders not firing, they take away power from two more good cylinders leaving you with effectively a two-cylinder engine. Check the compression on all cylinders also.
