I'd re-try:
If that won't work, did you try Radeon Software Crimson Edition Beta 16.2.1 for hd7450 and windows 7 x64 (click)?
- Remove the dust from inside your pc, especially from fans and air slits.
- Apply a new layer of thermal past/compound on the cpu.
- Clean the slot and the gpu card connector.
- Try to improve the fan settings and/or their position inside the case (or get better fans, if they don't suffice).
If that won't work, did you try Radeon Software Crimson Edition Beta 16.2.1 for hd7450 and windows 7 x64 (click)?