I trust that you tried to match the original RAM's specs as closely as possible.
Also try re-seating the sticks. Make sure they are completely locked into their slots. Even the old RAM - you can move things putting the new sticks in.
The most obvious cause would be a bad stick of RAM. You test this by running a program like Memtest86+ - Advanced Memory Diagnostic Tool
. Remove all RAM, then install one of the new sticks into Slot 1 and run the test for 3 to 5 passes. Do the same for the other stick. If you get any errors then you know which stick is bad.
Another cause could be a defective RAM socket. But you need to know that the sticks are good to begin with.