large variations in a very short time can be due to
… DL power control + baseband frequency hopping : “bcch frame” rxlev will be high, whereas other frames will be low level.
… a faulty TRX or a faulty ANC (amplifier)
Now, you have to tell us what do you consider as a “large” variation (how many dB ?) and in what kind of time intervals they occur. Does the MS ping-pong between different cells ?