Go to the router interface, login>>setup>>wireless settings>> verify that Enable Auto Channel Scan is UNCHECKED, pick a good channel like 1 or 11. I think some really old adapters will NOT do channel 11, but that is really the best channel away from the neighbors cordless that also resides in the same frequency range. I see above you tried 11, then go for 1, 6 is the worst!
Did you do a hard reset after the firmware upgrade (a little button on the back of the router)?
Also, do you have mixed adapters(computers) on the wireless, B,G,N, or just one flavor like G? If your computers are all the same like G, set that also while you are in the wireless setting menu, to G only or G & N only.
Under tools>> time, is enable daylight savings time checked, if so uncheck (I have no clue why), also while you are there, set the correct time "copy your computers time setting".
Honestly you should have stayed with 1.21, I switched back and forth 3 times between 1.21 and 1.34, 1.21 was the best!
If you search, there is a thread on how to down grade from 1.34 back to 1.21. When I got my router it had 1.32 in it, it was useless and I went to 1.34, it was still pretty bad! I had 1.21 up and running for several days with no problem before I sent the router back!