idleing irraticly and dosnt always start. please help

hi all. new to this site and hoping that some one can help. iv just gone and bought a mk1 gti 1800 cabelet and hoping to have some fun  with it but got problems.realy having trouble with starting at times. iv changed plugs, leads.new battery. the problem is that it isnt all the time and is usally after its run a while , normaly happens wen stopped at a shop or in the petrol station and just wont fire up. iv had a guy who recovered me the last time i broke down say he thought it was a problem with my earth and iv now  sorted that and thought id sussed it until thursday when it all started again.when she does start up and is on tick over it idles erraticaly between 800-1600.. iv tried adjusting it to 1000 but seems to have a mind of its own. i no that i have a spark and fuel getting there so nothing as obvious as that…any ideas…sorry for the ramble and hope this makes sense.