Back to the OP - I have noticed some of this also on 4.2.1. as I type this on my trusty thinkpad I have 4 bars [my home AP - multi routers one AP]. On my very humble old Optimus P509 'droid I get 3 bars - no issues. on the nexus laying on the table in EXACTLY the same location, I get 1 bar, which will 'wobble' to zero, back to 1, etc.
This location is line-of-sight to the stronger of my routers [external antennae].
? I don't have google currents.
any ideas?
I will say I have seen worse

.... my wife's galaxy S 4G seems to pick the time and place at which it decides to receive wifi.... a problem I need to work on some day - that thing is terrible about connecting to wifi. my little optimus never complains about anything. connects like a signal-vacuum
UPDATE: more weirdness. put our ipad mini in the same spot and it gets 2-3 bars.
On the Nexus 7 I disabled and re-enabled wifi after a power-cycle boot. here's something interesting. if I change from landscape position [sensor picks it up] to portrait mode, the signal will change to the stronger, but it will drop the connection, re-authenticate, get connected again. reverse the procedure shifting to landscape mode - sensor flips the display, signal drops, disconnects, tries to reconnect/authenticate, secures "connect" at very low signal. I tried this multiple times holding the device [handwise] in different ways thinking maybe my hand/fingers in contact had something to do with it - the hand position seemed to make no difference. landscape mode bad. portrait mode not as bad. disconnect/reconnect, etc. No wonder there is "lag" going on if its cycling the connection every few seconds.
What CHANNEL does the Nexus 7 like?