Netstumbler gaps

I'm looking at the Netstumbler response graphs for one of my Netgear
MR814v2 routers and I see many gaps in the response where the signal
level drops from -45 dBm to -85 dBm. The gaps last for a second or two
and then the signal looks fine for a few seconds. The pattern (not
perfectly periodic - the time at strong signal varies from about a
second to 5 seconds) doesn't seem to clear up no matter how long I let
the router run. Netstumbler shows a purple line indicating loss of
connectivity at the start of each gap. The gaps don't disappear even
when I put the client radio right next to the router. I tried
resetting the router and also disconnecting everything from both the
WAN and LAN ports and it still did the same thing. I ran a speed test
at dslreports while connected to the net through the router and my
speed is fine (almost 5 Mbps). The gaps, if they are real, don't seem
to be affecting performance. I took a look at the signal from another
identical MR814v2 router using the same client radio and I don't get
the gaps. Any ideas on why this is happening?

Thanks,
Bruce