Good for you.
There are several problems when one attempts to revive a Radeon HD6970M. Assuming that's just been accomplished, often no attention is made to the fans. A quick blast of air from an air duster canister can imho only be partially effective, and in my experience it doesn't dislodge possible heavy accumulation of debris from the exit guide grill directly behind the fans, as this cannot be observed unless the fans are removed - a simple task once the screen is removed for internal access.
The use of higher pressure air from a compressor may be sufficient to dislodge debris accumulation from the exit guide grill in-situ although I've never tried this, always preferring to remove the 3 fans to ensure absolute cleanliness.
The other MUST as already mentioned is to permanently remove the optical drive, and also replace the very hot running HDD by an SSD.
With an SSD, no HDD or optical drive, clean fans and fresh re-application of good quality thermal paste on the CPU and GPU, one can be assured of a far cooler running iMac.
One final thought. The Pram battery should also be changed whilst the iMac is open. I've found it difficult/impossibe to obtain the recommended BR2032 cell battery, even Amazon here nationally don't stock it. The readily available CR2032 'alternative' may be fitted, although the last time I checked the specs gave the BR2032 a 200mA rating opposed to 220mA, and a slightly higher max operating temp (80°C v + 70°C) which presumably should give the BR2032 a lifespan benefit.
