Optiplex startup problems

I purchased an Optiplex in May and it has always been frequently slow to start. Sometimes it boots normally, other times it sits on the Dell welcome screen for 10 minutes then boots and other times it just shows a blank screen - it doesn't even show the Dell post screen but if I leave it, it eventually starts often after an hour. Dell advised me to reinstall Windows after an online discussion and it made no difference. What to do about it?