Should I stay and forgive or leave and forget?

I've been married for 10 years. 3 kids 8,7,5. My marriage has had it's up and downs. He has been unable to find employment for the past year and a half putting every on me. And I have to admit it made me a little bitter and snappy working 70 hours plus a week and on call 24/7. But about 3 months ago I took a step back and realized that even though he wasn't working at a job. He was taking care of the kids and the house and doing alot of thing to allow me to rest when time allowed. So I made a conscience effort to change my attitude and get my marriage back on track. I started telling him the house looked nice...he looked nice..thanks for letting me get some rest and actually started having a good life in the bedroom again (before I would go a month with no action. I was to tired). Anyway he and I both were commenting constantly on how our marriage was better than it had been in YEARS. Anyway long story short.After a awesome romantic night out..I heard my cell phone ring(the one I leave with him) and headed to the back to answer and it. He pulled it out of his pocket and said "wrong number" twice very nervous and fast and hung up. I knew right then something was strange. I called the number back and they answered but said nothing..Until I said "Hello" and they hung up and wouldn't answer again. About a hour later I called from my other cell and a Female answered. When I spoke she hung up and never answered again. He kept swearing on his mother that he had not cheated but would not make the same swear that he didn't know who's number it was. Finally he said I won't swear to that because "I know who it is". Supposably someone had dropped a phone number in the penger window at a store and said "hey my friend likes you give her a call." I asked him how many times they had talked and what the intent was. He said about 5-6 and he had no idea why he called..He said he made the call then didn't know how to get out of it. He said that the reason she had called when I was there was he had just called her and left a message that he had made a mistake and please don't call but she must of not checked VM and just called right back. I checked phone records and the conversations were 1 or 2 minutes long with the exception of one 12 minute call. It also checked out that he had called her at 11:14 and then she called back at 11:16 (the call I caught). During the whole mess I told him that if he had left the message he needed to call back in front of me and reiterate what he had said on the message. Which he did. I am having a really hard time with this and don't know what to do...He is very apologetic and said that the only reason he can think of that he called was to see if he still had it in him. (he is 13 years older than me). I work to much and to hard to play detective I either have to trust and move on or just move on. Am I making to big of a deal out of this or not a big enough deal...Will it happen again. He is like a dog with his tale tucked right now...Please help any advise is appreciated.