FCS stands for Frame Check Sequence. Each PPP packet has a checksum attached to ensure that the data being received is the data being sent. If the FCS of an incoming packet is incorrect, the packet is dropped and the HDLC FCS count is increased. The HDLC error values can be displayed using the show hdlc command.
If your link is bad (or if your serial driver is dropping packets), you will see the occasional FCS error. This is not usually worth worrying about although it does slow down the compression protocols substantially. If you have an external modem, make sure your cable is properly shielded from interference - this may eradicate the problem.
If your link freezes as soon as you have connected and you see a large number of FCS errors, this may be because your link is not 8 bit clean. Make sure your modem is not using software flow control (XON/XOFF). If your datalink must use software flow control, use the command set accmap 0x000a0000 to tell ppp(8) to escape the ^Q and ^S characters.
Another reason for seeing too many FCS errors may be that the remote end has stopped talking PPP. You may want to enable async logging at this point to determine if the incoming data is actually a login or shell prompt. If you have a shell prompt at the remote end, it is possible to terminate ppp(8) without dropping the line by using the close lcp command (a following term command will reconnect you to the shell on the remote machine.
If nothing in your log file indicates why the link might have been terminated, you should ask the remote administrator (your ISP?) why the session was terminated.