IBM Intellistation power problem

I have an old IBM Intelliestation E Pro desktop machine. In fact I have 2.
Model 6893

When you first attatch the kettle lead to the back the machine turns on
immediately without touching the power button at the front.

This happens like this on both machines and the machine wont boot or show
any pic on the monitor.

In the past I have put the power off using the front button and when you
switch it back on everything works fine.

At least "I think" thats the way I done it. What I do know is there was a
specific thing I had to do to get it working and now I have forgot.

Today I switched on one machine after connecting the psu kb and m. As usual
it switches on and the cd rom spins etc but no boot. The problem this time
is that I cannot switch it off using the power button at the front of the
machine. If i press it in and let go it just stays on with the green light
showing. If I hold it in for 6 secs it powers down but as soon as let the
button go it powers up again. I tried letting go really quick as soon as I
hear the machine powering down but no joy it still starts up.

I tired switching the mains on and of at the wall socket but no use.

Anyone got any ideas? Like I say I KNOW I have done this before and had the
exact same problem but I always got pout of it I just cannot remember how.
Either that or I am remembering correct but something else is wrong. All I
know is that if I can switch it off with the power button then it WILL start
up properly.