I've been trying to target weld vertices on a sphere created in the vertex modeler. Some of the time I can select the vertex I want welded and sometimes I have to try 10, 20 times to select the first vertex. Once the first vertex is selected, I can usually select the second vertex. I thought that I might be doing something wrong during the selection process.  I have no problems selecting vertices using other tools. I am using  my old slow computer. It's a Pentium 4-2.8GHz with 1GB of memory.