Hi jersey, sorry you're here.
I replied to this chain on everything you should do first (10 things to do immediately - not all of them apply until you have hard proof (like, STD tests aren't required if he hasn't gone through with it yet);
http://www.infidelity.com/forum/Day-10So-confused-t2672.htmlPay special attention to #10 - get a key logger! Consider getting a VAR (@ $50) and put it in the car or wherever you think he's having these secret conversations. Definitely keep your eyes open and stop it from going any further as soon as you have proof. He'll just deny it until you have proof - but once you have proof, you need to hit him with it hard. And if she's a co-worker, he needs to quit that job immediately. It's impossible to rebuild the trust (and end the affair) if he still has contact with her.
How did you work through it last time? What did you do? Did you figure out what made him vulnerable and is he doing everything he can to prevent it from happening again? [Obviously not, or you wouldn't be suspicious... sorry].
I hope you don't find anything, I hope he's just in the initial stages, but you need to work through it right this time. If it's starting again, you didn't work through it last time - he just ended that affair and you both let it go over time. That doesn't work.