"There is hardly any paste left on the CPU/heatsink join. Could this be an issue?"
You bet it can be an issue. Clean all the old stuff off the CPU and the heatsink with alcohol and apply new thermal compound and make sure the heatsink is correctly installed.
Good luck
Biostar TA790GX A2+ 6.0
AMD Phenom X4 9750 CPU.
4 Gig DDR2 Memory.
ATI HD 5450 PCIe Video
ATI HD 5450 PCIe Video
500 Watt P.S.
LG W2241T Widescreen 22" LCD
ViewSonic VA721 17" LCD
Envision 17" LCD
2 LG DVD Drives
Floppy Disk Drive
Maxtor 120 Gig Windows 7 Home Premium 32 bit
Gateway NV5378-U Windows 7 Home Premium 64 bit
Acer Aspire V3-731 Windows 7 Home Premium 64 bit