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
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Rayleigh fading shouldnt happen too much now if the network planner knows what he is doing.
Could be lots of things, report it is your best option.
Also depends if you are on 3g or 2g phone wise.
If you are on auto some days you might be on 2g and get a very good signal ie full bars, other days you may be on 3g, which is sh1t inside buildings. Therefore you get the impression that the same signal is going up and down.
It should handover but data fill isnt the greatest on some Telco's. 2g to 3g and vice versa.
There could be a problem with the site or it could be congestion or interference from another site if they have fiddled with frequency reallocoation.
An ice cream eating layabout may turn up and check it out for you
ps it could be your handset too...try another if you have one.