Display pixilates after running applications.

Running LinuxMint 15 in standard mode: Display perfect initially. After running an application e.g. libre office or firefox, the desktop background becomes pixilated and gets worse if open a second application- at which point desktop display not usable. There isn't pixilation inside the application!
Problem not there if start Mint in "safe" mode ( but this gives warning that acceleration not used and display resolution is poor).

Graphics: NVIDIA Corp G86 [GeForce 8300 GS]
Processor AMD Athlon(tm) 64X2 Duel Core processor 5600+ X2
Disk 133.3 GB. Memory 3.9 GB.