For malformed packets, these can be a Wireshark bug or a real malformed packet. Use other tools to isolate the problem. Suspected bugs can be reported on the Wireshark website.
When you see many malformed packets of checksum errors, it is probably because of offload or dissector errors. Networks with more that 1%-2% errors of any kind will cause many other events (retransmissions, for example) and will become much slower than expected; therefore, you cannot have a high error rate with a functioning network!