Unable to Connect to Server iPhone 4

After about 3 weeks of perfect service with the iPhone 4, I began to receive an error message about 5 miles from my home that unable to connect to server. Would be listening to satellite radio and would get a seaching for the signal message. Had no network service at home - thus no email, internet, etc. However, the three iPhone 3s in my household had PERFECT connection to server.
Phone works perfect once I get to work or beyond the 5 mile zone from my house.
Contacted Customer Support & they advised to return the phone as within the 30 day exchange period - which I did. Apple diagnosed & noted numerous server connection issues & replaced the phone no questions asked.
On the way home with new phone, lost service at the exact same location. Contacted Customer Support & was advised to take phone to Corporate AT&T Store to have them replace SIM card - which they did. On the way home - with new phone and new SIM card - lost service at the exact same location.
Contacted AT&T and was advised no reported service connection issues in my area. However, after being transferred up the chain of command several levels, was advised that they did in fact have an issue with a tower in my area which might be causing the loss of singnal & inability to connect to the server. Repairs were expected to be completed in several days - which they were and my service was restored.
Until this past week - began experiencing the same issue of not being able to connect to the server in the same area as before. Family from out of state vistited with two iPhone 4s and they were not able to connect to server - yet the iPhone 3s continued to work fine. Contacted AT&T this morning and was advised no reported problems with service in my zip code area.
Any input or comments would be most appreciated. Not a tech expert but cant undersatnd why phone works perfect 5 miles & beyond from my house - yet I lose abilty to connect to server when I approach this zone on way home.
iPhone 4