screen problems after hd exchange

[fvjfvj]

I had to exchange the HD on my daughters mid 2007 iMac. Everything seemed to go reasonably well, until I swiched the iMac back on.

After the first grey screen the picture of the install disk looked awful, the colours where all messed up (more like 16 colours), there where vertical stripes in some parts of the display, and whenever the screen turned black, it showed patterns of green pixels.

Apart form the ugly display, the system was responsive so I went ahead and reinstalled Snow Leopard from the Install DVD. Having done that I went through the usual motions of P-Ram reset SMC - Reset, all to no avail.

Since it was saturday night I unplugged the machine and decided to take it to one of our certified repair shops on monday. Monday evening I went there told my story, the attendant listened politely, took the iMac, plugged it in, and .......

No problem whatsoever, brilliant clear colours, no lines, no patterns, no nothing, when asked what the problem might have been, or what I did wrong the guy just shrugged, and could no deliver an explanation.

So my question, has anybody experienced something like this before, or has an explanation as to what happened, or why, and why did the problem disappear all of a sudden ???