If you shift your service to EDGE, or put the device into field test mode, you will see that your service is pretty consistent.
The signal strength bars are highly erroneous, especially on 3G. The bars will indicate you have 5 bars when in reality, you have no signal.
Field test mode replaces the bars with decibel values. There is a sliding scale where values less than -90 db equate to less than 1 bar of service. Best service is reached around -70 to -75 db. The iPhone's radio is picking up a lot of noise and scattered signals.
Here is an example link to help understand the issue. There are many others out there. Bottom line, move your phone to a different location where you see better signal strength.
ask.metafilter.com/60227/What-do-cell-phone-reception-bars-mean]What do cell phone reception bars mean? | Ask Metafilter