Hi Bam welcome to Talk Graphics. First thing turn off the viz. of your text that can be done again or enlarge to suit full size drawing. Have a look here for the rest of your work: http://helpx.adobe.com/photoshop/usi...esolution.html
Also have a look at this one gives good info to see if you can improve the resolution. http://www.graphics.com/article-old/...ution-and-size
There are a few plugins that you can buy to improve res. which you can trial for 1 month which also might help your problem : http://www.dxo.com/intl/photography/...FfMgtAodsXgApg