I recon it might be the viscous coupling in your clutched fan. If you can hold it whilst the engine is running its stuffed. If it isn't stuffed, you may just cut your fingers off.
Try spinning the fan with the motor off, if it spins fairly freely its dead.
The symptoms make sense for that though, when your driving the fan doesn't matter, and when you rev it up it probably gets the fan spinning enough to pull some air through the radiator. But when sitting the fan probably doesn't spin fast enough to pull enough air to keep it cool.