Very strange and annoying problem with my A500 PSU

[AmigaMance]

Hello.
 I'm using an original A500 PSU for my Amiga 1200 and it has served me well well for many years.
 Just 3 day ago, i experienced a very strange problem:
When i switch on the PSU, the power led of the Amiga 1200 flickers like MAD (not dimming like in a guru, but flashing very fast) and i hear a constant noise from the PSU that sound like an electric discharge. My Amiga doesn't boot of course.
 If i switch ON the PSU without plugging it to the Amiga, i don't hear this clicking sound, thus i suspect that it's a problem from the part of the circuit which is beyond the secondary coil of the transformer.
 I thought that dirt was causing a capacitor to discharge rapidly, so i opened it and cleaned it with a big amount of alcohol and cotton pads. After i cleaned it up, the PSU was functioning ok again and so did the Amiga!

 After a whole day of Amiga experience, i switched off the PSU and went to sleep with the confidence that i have solve the problem...
 The other morning i switched the PSU and here it was!! Rapidly flashing led and the discharging-like sound from the PSU.. :-(
 I followed the same cleaning procedure.. PSU started to work ok again.. went to sleep.. the other day same problem!!, And so on, and so on, and so on.
 I feel like i've been trapped into a time loop. What could be causing this?

[AmigaMance]

Maybe it's the power switch itself? The contacts could be worn after years of use, causing an bad connection.

 I don't think so, because that would cause the problem to occur even when i have not connect the PSU to the Amiga.

[AmigaMance]

@DrDekker and golem

 Yep, it's the same PSU.. :-) I think i have enough of it and i will exactly what you said. I THINK i have found a temporary solution though: I plugged it, in my spare Amiga 1200 and it worked ok. Then i thought.. let's open my Amiga and clean the area around the power plug.
 Well, it is working right now but who knows for how long. I will hack the cable and attach it to an AT.

 Let's see if the other problem vanishes as well.

[AmigaMance]

Btw, the AT PSU that i'm holding right now doesn't have a Watt rating but the label says:
OUTPUT:  +5v  20/23A
        +12V   8A/9A
         -5V   0.5A
        -12V   0.5A

 Is this enough or i will have to buy a more powerful one? My setup in not too bloaded. (No PCI, USB boards)

EDIT: and OUTLET:
 120/240V ~ 60/50Hz, 1/0.5A
The power network produces 220V, 50Hz here.

[AmigaMance]

motrucker wrote:
You are really playing with fire, using a PSU that doesn't act right. Remember that a psu can easily give the computer too much voltage/amperage/etc., and kill the computer.
Get a new psu quickly!!
 

 Well, i have a AT PSU right here. That's why i typed its specs and ask if it's ok or if i should buy a more poweful one.

[AmigaMance]

 Neat! Thanks for all the replies.

[AmigaMance]

Ok.
 I have cut off the plug from the A500 PSU and i'm ready to connect it with the pc one. Sorry if this is a stupid question, but i haven't done it before:
 The cable of the A500 has 4 wires + a shield/ground cable that surrounds the other ones, while the cable from the PC PSU has 4 wires. (Actually it has 2 plugs with 4 wires and 2 plugs with 6 wires!)

 My question is, what do i do with the extra shield wire? Do i just short-circuit it with the ground wire?

[AmigaMance]

 Thanks DrDekker.
 I will leave the shield wire unconnected and maybe try to attach it to the casing later.