i have exactly the same problem with a PA4020P kit,
tried with the 2 firmwares avaliable to this model on the site, and no luck.

500A-1.1.0.844-01-20120919-FINAL is the firmware for my TL-WPA4220.
500A-1.0.0.3370-01-20120309-FINAL is the firmware for my TL-PA4010.

Can't update the firmware and I am having unstable connection problems.
Constantly pinging the router to keep the connection alive seems to solve the problem (but that is not a real solution).
I just sent an message to support, let's see what they say.

Hi,

I have the same issue with TL-PA4010P !
After upgrading with the newer firmware (2013) the PowerLine Utility shows the same original firmware (2012 final), but newer firmware should have solve this issue.
Keep pinging not a solution at all...

Hey TPLINK SUPPORT !!

Where is the support ? Where is the solution and answer for these questions?

The item is unusable - network always getting slow or broken connection occured.

Any suggstions?

Thanks in advance
Beta

Hey TPLINK, is there any solution?
Firmware upgrade is not possible (successful but the version is not changed)
The link is always crashed or getting slow...

I have the same problem with my PA-4020P KIT

I cant update my firmware to the newest.

The utility always show 500 a-1.1.0.844-01-20120919-final as my firmware....

Read the explaination about upgrade here.
http://forum.tp-link.com/showthread.php?80292-TL-PA4010-drops-multicast-packets&p=158193&viewfull=1#post158193