Is this a 3G or standard 2G signal. 3G has a massive limitation in the amount of users on a single cell site. It will always favour a stronger connection i.e someone closer, or different times of day yield different results.
Environmental factors can affect it like weather, trees, obstructions in places at certain times.
If the mobile phone is used in urban environments, multipath fading is also contributing to attenuation of the signal. The signal can be reflected once or several times between buildings. If the signal from the GSM radio station is reflected from two different buildings, the resulting signal may be amplified (if the two signals are in phase) or it may be attenuated (if the two signals are not in phase). Explains why where signal is VERY poor you can sometimes get one bar at a certain spot.
Different phones have differing sensitivities with regards there internal antenna as well. I doubt anything is intefering with it, as the Operators have a licence to operate on a section of the 1800mhz band for GSM/2G and a section of the 2100 band for 3G
Could be maintenance on the local cell site or lastly Mast switching it may for someone reason not communicate with the closest mast (It should) and hand off to a different mast further away