I would say I am a devout Muslim but sometimes during encounters with my wife I imagine copulating other women in place of my wife, is this sinful? If so, how can I stop it?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions. It is forbidden for the husband to imagine, while copulating with his wife having sex with a different woman as it is forbidden for the wife to imagine, while copulating with her husband having sex with another man. The Maliki , Hanafi Hanbali and some Shafi'e are all of the above opinion. Other Shafi'e believe that it is lawful. But the first opinion is safer and more appropriate for the Muslim. As to how to get rid of such thoughts, it is to fill ones mind with one's wife. Allah Says (interpretation of meaning): {…And know that Allah knoweth what is in your hearts, and take heed of Him: and know that Allah is Oft-Forgiving, Most Forbearing.}[2:235]. And anyway would you appreciate that your wife does the same to you? Allah knows best.