Client key exchange will contain:
length (2 bytes) --> Y_C (in plain text)
I have implemented TLS in Java and I follow the same structure and works fine for me.
Do I need to sign the returned Yc?
No there is no need to sign the client DH public value, it is transferred in plain text.
You can take a pcap and check whether same values are being transferred in the packet. Also if GNU TLS has logger for printing the Y_C received, then you can check if proper data is being received.
If in case you still getting different Pre-Master secret then there seems to be some issue with the logic of generation of secret.