My husband and I have been married for 18 years ...

Well, first off... you know he has a problem. Try talking to him but don't confront him sternly. Just be calm and see what happens. If nothing happens, get your husband some help. Just don't become his enemy. He is going through something weird right now and he needs someone to pull him away from it. You must be there...through thick and thin, better or worse. IF however he doesn't stop...leave. You deserve better. If he spends more time on the computer than you...leave. 

Hi Ann,
Mickey (welcome to Yedda !) gave you a very good answer (2 thumbs up).  I would like to add: It is clear from your notes that you love your husband, don't want to break your marriage, and want to help him, but at the same time can NOT tolerate his cheatings.  The best way to attend to your (his) problem is heart-2-heart, open and deep discussions. If you feel that you won't be able to do it on your own than better get a proffesional experience marriage counselling. 
Best regards,