Not sure how to help you. If the transfer speed is high enough the encryption can consume a lot of CPU.
Though, I do not understand why you need to restart your machine to resolve this.
Anyway, have you considered running WinSCP process with lower priority or decreasing transfer speed?

We are using WinSCP 5.1.5 to transfer files from our Windows 2008 R2 x64 to other remote servers.
The WinSCP.exe*32 process consumes aprox 50% of the CPU.
If we initiate a second transfer in parallel, it takes up the rest of the CPU.

Details:
File Protocol is = SFTP,
Port 22
Preferred SSH protocol = 2
Encryption Options = Default value

Is there a way to decrease the amount of CPU used by WinSCP?
Any recommendations will be appreciated.
since we have this issue, we have to restart the windows server every time when it happens. it's interrupting our operation process.
Please help us to solve this issue.