Hi all, think i got it sussed (sort of).
Downloaded my m/b manual and it only supports up to 2100 (133mhz) ram. Although it recognises and uses the 2700 (166mhz) ram, it only can do it with the FSB set to 100. Setting it higher creates a problem somehow with the graphics card. Not too certain how this is, but i guess its my fault for having an ancient mobo
Did a test by taking out the 2700 ram and setting the FSB to 133 and it ran fine. Then i tried the 2700 ram with some 2100 ram in another computer with better mobo (supports up to 3200ram) and that had no problems either. Will try Nubius' idea of using the 2700 on its own just to confirm.