IMac screen going dim . . .

Lately my older imac screen has been somewhat dim when waking from sleep.... only happens sometimes, seems to be normally bright at the bottom, growing dimmer the further toward the top you look .... if I restart, everything is fine again.
Hardware problem? Seems unlikely since it seems unrelated to how long the computer has been on, it's immediately fine on restart. Any ideas appreciated . . .

Some problem with my iMac. Took it back to the Apple Reseller, they 'repaired' it after 3 weeks. Got it back, still the same problem. Called them and they said is was a normal buzz which the inverter card(?) makes, like the fan and the HD.

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