Are you taking the motherboard out of the case and placing it on a hard flat surface when you remove or install the heatsink? It's a good idea to have the motherboard on a hard flat surface to support it and keep it from bending too much when working with the HSF, video card or RAM. Too much bending can cause the solder joint on the motherboard to crack, or the circuit traces on the motherboard can also crack.
I would still test the RAM with Doc Memory or MemTest86, just to be sure.
I never heard of Speeze heatsinks...do they get good review? Is it fairly large or does it just cover the CPU socket without too much overhang. It could be too small for your CPU and unable to cool it properly. Having too much thermal compound can prevent good thermal transfer. You only need a very little amount for it to work properly.
Do you have a good quality power supply? Or is it a generic one in a cheap case? If it's a DEER brand power supply, consider replacing it ASAP...those are very poor quality and can damage your other components if it dies.