I've been having the exact same problem and search for a solution. Eventually I found a few apps in the app store (I never normally look for software there!) that claim to be able to reconnect network drives after sleep:
Drive Mounty - I tried the pro version of this (£2.99) for a few weeks and it seemed to mostly work ok but suddenly started hammering the CPU and failing to mount the drive and also adding random suffixes to the mount point. The free version is restricted to 1 mount point and limited remounts. Was promising at first but sadly became unreliable.
Auto Mounter - Currently trying this (£9.99) with the Pro options pack too (£2.99). Looks good so far and has lots of features and the ability to create rules. Can mount drives in /Volumes using a separate helper utility that overcomes the App Store sandboxing. It's a shame there is no free trial version available.
Drive Mounter - Not tried this because Auto Mounter is holding up well so far. Has a free version too to try.
If you try any of these let me know how they work out.