Welcome to the wonderful world of streaming digital audio.
IMHO,
the weak link that causes the dropouts in the chain that you describe as "laptop (running Windows) with external hard drive / wireless to Airport Express / Audioquest Forest optical cable / Cambridge Audio DacMagic"
is the wireless link between your laptop and the Airport Express.
Wireless works great for bursty file transfers like when you request a website and you just read some text. While you are reading, the wireless, your storage, and your computer have some time to assemble the next sentence/paragraph. But music (and videos) do not work reliably over a bursty transmission mechanism.
A cheap experiment you can perform is to
connect your external hard drive to your Apple Express via USB. You will need to tell your software (iTunes, Foobar, etc.) that the files are on a hard drive on your network. You may need to configure your network to "share" the external hard drive attached to the Apple Express on your network. This can solve your problem without extra expense but with some playing around your network configurations. Disclaimer: in case what happens is the files are sent from your hard drive to your Apple Express to your computer then back to your Apple Express to your DAC, this may in fact exacerbate the problem. Hopefully, your hardware and software were designed well enough that the files are not streamed via this roundabout route.
I have tried multiple combinations of getting computer and networking equipment to act as music servers. My current chain is:
-FLAC/ALAC in a Network-Attached Storage
-connected to an Airport Extreme network (2 of them actually, in an extended network).
-MacMini running (a trial) Amarra sending out a music stream
-via an asynchronous USB-SPDIF converter
-to a DAC.
-Control is via an iPad
I do get dropouts once in a while especially when playing the sample high res files I download from places like HDTracks.
When I find time, I will move my music (and video) files from the Network-Attached Storage to an external drive with USB 3 or Lightning connectivity which connects directly to the MacMini. This route makes sense to me because I already have the various bits and bobs to set things up like this. If I were doing this from scratch, I'd get something like a Naim ND5 XS and an external hard drive.
It is certainly doable but you will need to get into all sorts of configuration and troubleshooting not just upon initial set-up but also when there are firmware updates, software updates, etc. This is great if you enjoy this sort of thing but I can certainly understand how blood pressures can rise quickly for those who do not enjoy tinkering with their computer hardware and software.
So an alternative is to use devices designed specifically for music. I believe Sonos was already mentioned. Most of the "audiophile" brands have some sort of alternative / competitor to something like the Naim ND5XS / NDX / NDS. If you go this route, you will still need to somehow have a hard-wired connection between your hard drive and your streamer. Net, the only thing that can go wireless is how you control your rig (using apps like Apple Remote, Plex, JRiver, etc.).
I hope this helps!