Dimension 9150 (RAM) Memory connector problem HELP

Diagnotic Lights 3&4 come on which the manual tells me means "Memory modules detected but a memory failure has occurred". I have two existing memory modules. I have followed the manual advice and have tried remove them and re-inserting them and have found out that both cards still work fine but if either of the cards is put into slot DIMM_1. the computer will not start up and I get the diagnotic lights 3&4. However both cards can make the computer work if either are installed into slot DIMM_2. Does anyone have any idea what else I can do or does this mean that DIMM_1 is broken and will need to be replaced?

Many thanks:emotion-7: