hello paul,
a signal level is always unstable, because of the multiple reflection of the waves : sometimes the signal is stronger than real (because of phase addition), other times the signal is weaker, due to phase cancellation.
The variation can be of +/- 10dB in a city, even if the MS is static.
You will notice that moving out of the city in an open area, the signal becomes a little bit more stable again.
Cheers
pix