Volvo Repair: 2003 Volvo S40 Wont Idle, 2003 volvo s40, volvo s40

Question
Several months ago my S40 wouldn't start unless I hit the gas and kept the RPMs up.  After that I had no troubles until recently.  I tried scrapping the oxidation from the ground lug on the battery and that worked.  The car ran fine for a couple weeks.  It then wouldn't idle again, I measured the battery to be at 12.5V with the engine off, 14.2V with it on.  I bought a new battery and it ran fine for a couple days.  My friend said it likely wasn't the battery but instead the wiring, so I plugged the old battery back in and it ran fine for a couple days.  It happened again today, so I put the new battery in, but it didn't fix it.  Could this be a corroded 12V wire to the ECU from the battery?  Thank you very much for your time.

Answer
Scott, what I have seen on the S40's and S60's is a bad starter relay.  When you mean doesnt start, Im assuming you mean the start motor will not engage.  Is this correct or will the car crank and just not run?  Need to be a little more specific. Is the check engine light on? Roger