Airport issues after 10.5.8 on Macbook 5.1

Question

After upgrading to 10.5.8 I have experienced several problems with the wireless connection.
I can correctly list the wifi networks, but I always end up with a self asigned IP. It won't ping the router, giving "no route to host" or timeouts up to 14000ms.
Sometimes (1 out of 4?), when I've been working with ethernet and switch to airport, seems to work properly for about 4-5 minutes. Then, both when running on battery or plugged in, the performance drastically drops until it becomes unusable.
I have tried all the solutions proposed to the similar problem (wifi issues when running on battery) with no results. The upgrade apple released didn't run for my hardware. I don't know if it's the same bug, but I hope this helps to identify it.
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Hardware specs (translated from spanish):
Model name: MacBook
Model ID: MacBook5,1
Processor name: Intel Core 2 Duo
Processor speed: 2.4 GHz
Wireless adaptor: AirPort Extreme (0x14E4, 0x8D)
Firmware version: Broadcom BCM43xx 1.0 (5.10.91.21)
Wireless channel: 6
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Ping once that worked right after unpluggin ethernet
[jacobo@scorcho-2:~] $ ping 192.168.1.1
PING 192.168.1.1 (192.168.1.1): 56 data bytes
64 bytes from 192.168.1.1: icmp_seq=0 ttl=255 time=16.113 ms
64 bytes from 192.168.1.1: icmp_seq=1 ttl=255 time=4.297 ms
64 bytes from 192.168.1.1: icmp_seq=2 ttl=255 time=5.439 ms
64 bytes from 192.168.1.1: icmp_seq=3 ttl=255 time=5.544 ms
64 bytes from 192.168.1.1: icmp_seq=4 ttl=255 time=4.263 ms
64 bytes from 192.168.1.1: icmp_seq=5 ttl=255 time=4.475 ms
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Usual ping
[jacobo@scorcho-2:~] $ ping 192.168.1.1
PING 192.168.1.1 (192.168.1.1): 56 data bytes
64 bytes from 192.168.1.1: icmp_seq=0 ttl=255 time=11320.400 ms
64 bytes from 192.168.1.1: icmp_seq=1 ttl=255 time=10322.923 ms
64 bytes from 192.168.1.1: icmp_seq=2 ttl=255 time=9323.677 ms
64 bytes from 192.168.1.1: icmp_seq=3 ttl=255 time=8331.722 ms
64 bytes from 192.168.1.1: icmp_seq=4 ttl=255 time=7603.590 ms
64 bytes from 192.168.1.1: icmp_seq=5 ttl=255 time=6606.180 ms

Answer 1

After doing a Software Update, AirPort has been having serious connection issues. It cannot find wireless networks where the *Wireless SSID Broadcast is DISABLED*. My iPhone finds the networks perfectly, every time. I am forced to input the network manually every time through "Join Other Network...", even though the network is saved on the Advanced list in Network Preferences.
*Here are the specs of my MacBook Pro:*
Model Name: MacBook Pro
Model Identifier: MacBookPro5,3
Processor Name: Intel Core 2 Duo
Processor Speed: 2.8 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 6 MB
Memory: 4 GB
Bus Speed: 1.07 GHz
*The router I am connecting to is...*
Linksys Compact Wireless-G Broadband Router
Model WRT54GC
Firmware Version: v1.05.7
Mode: Mixed
Wireless Channel: 06
Security: WPA2 Personal
Encryption: AES
Everything is pretty much defaulted to the default settings for the router. I have also another Linksys Router that is having the same problems. These used to work before perfectly BEFORE the MacBook 10.5.8 updates...
*Previous Methods in Trying to Resolve Issue:*
-Deleting AirPort connection in Network Preferences and re-adding it
-Deleting the "com.apple.airport.preferences.plist" file in /Library/Preferences/etc.
I'm at a loss here... any help would be greatly appreciated.