do the paperclip test and see what your fault codes are go from there
http://www.topbuzz.co.uk/info/fault_codes/fault_codes.htm#top
My 97 r had a similar probem with varying degrees of misfire.  It went to Vauxhall twice for diagnostics.  The said cam sensor and plugs both of which i replaced and although better still not perfect I then replaced the leads and misfire has gone but still got EML on.  Did the paperclip test and it shows MAF sensor so that is next - wish i had done this test 1st myself instead of paying the idiots at my local vauxhall garage!!!
                
 Leon
                
 February 2009