USB 2.0 Hubs hooked to iMac G5 can't seem to sustain devices ...

Question

about a month ago I noticed that my iPod and iPhone would occasionally disappear from my Device list in iTunes while plugged in. Both were connected to my computer via USB slots on my NewerTech miniStack v2. I plugged various devices into the USB slots on the Newertech and found that they would fail as well (such as, Turbo.264, iRocks 2.0 USB hub). I then tried plugging the devices directly into the back of my iMac G5 (except the iRocks hub) and they seemed to work fine. I believed I had determined (or so I thought) that the miniStack was failing me (curiously enough, the firewire still worked on the miniStack).
Well I have since unplugged all USB devices from the miniStackand and am using the plain jane iRocks USB 2.0 hub. Unfortunately, as was the case when devices were plugged into the miniStack, devices have started to go missing (unmount, fail, whatever you want to call it). It is important to note that the devices never fail when plugged directly into the back of the iMac G5 (with the exception of the Turbo.264 which has once "failed" when directly plugged into the back of the iMac).
Just last night, when I plugged my iPod 5.5 80gig into the iRocks USB Hub to syn, iTunes told me the file system was corrupt and that I would have to restore the iPod. When I attempted to do so, iTunes gave the the following message: Unable to restore iPod, unknown error occured. Error 1418. I plugged the iPod into the back of the computer and, wouldn't you know, the file system was "no longer corrupt."
Am I looking at a rather large problem with my iMac or have I run across the incredible coincidence of having two USB hubs fail at similar times? Any help would be greatly appreciated.
oh, and the important info:
Hardware Overview:
OS X 10.5.6
Model Name: iMac G5
Model Identifier: PowerMac12,1
Processor Name: PowerPC G5 (3.1)
Processor Speed: 2.1 GHz
Number Of CPUs: 1
L2 Cache (per CPU): 512 KB
Memory: 2.5 GB
Bus Speed: 700 MHz
Boot ROM Version: 5.2.6f1
USB Bus:
Host Controller Location: Built In USB
Host Controller Driver: AppleUSBOHCI
PCI Device ID: 0x0035
PCI Revision ID: 0x0043
PCI Vendor ID: 0x1033
Bus Number: 0x0b
USB High-Speed Bus:
Host Controller Location: Built In USB
Host Controller Driver: AppleUSBEHCI
PCI Device ID: 0x00e0
PCI Revision ID: 0x0004
PCI Vendor ID: 0x1033
Bus Number: 0x4b
USB2.0 Hub:
Product ID: 0x0606
Vendor ID: 0x05e3 (Genesys Logic, Inc.)
Version: 7.02
Speed: Up to 480 Mb/sec
Location ID: 0x4b100000
Current Available (mA): 500
Current Required (mA): 100
USB2.0 Hub Controller:
Product ID: 0x0058
Vendor ID: 0x0409 (NEC Corporation)
Version: 1.00
Speed: Up to 480 Mb/sec
Manufacturer: NEC Corporation
Location ID: 0x4b140000
Current Available (mA): 500
Current Required (mA): 100

Answer 1

Miriam,
Thank you for your many responses. I have checked the outlet in so much as I checked other devices (stereos, tvs, etc.) and they seem to work fine. Yes, a power strip is used but again, and maybe I don't understand the difference in how USB uses power and Firewire uses power, but the miniStack uses both USB and Firewire connections. When set strictly to Firewire for the hard drive data transfer, the hard drives never "unmount" or dissapear; this seems to be solely a USB problem.
Now, outside of trying other devices, do you know any other way to test the outlet? I am at a loss but I guess I am grateful that no one has suggested that my iMac is dying lol.
Cheers