Large downloads failing (v1.3.7)

I have large downloadable products that are failing for various users. I have no problem downloading them and some others don't either, but I am having several people finding it hard to do. The downloads stop abruptly or they act as if they are complete when they aren't. Dreamhost says everything is fine on the server side and recommends upgrade zen cart, maybe it's a php code issue for large downloads. Can someone help? I'm really not very knowledgeable in this area.

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version: 1.3.7
PHP Version 5.2.17
I don't see any additional modules installed
customization is basic from what i see: colors, banners,etc.
dreamhost is our hosting company and there doesn't appear to be an issue there.

I did test large downloads, though I have like an 18MB download while most of the people with problems have in the neighborhood of 3MB or so if that.

this is what the dreamhost tech support said, "

I'm sorry I wasn't able to pinpoint a flag as to the cause of the issue
(I definitely saw it in Safari and Chrome), but testing is showing it's
likely an issue with the scripts trying to send such large files. I'm not
finding any information that makes it browser-specific, since I had
success in both Firefox and RockMelt. You may want to see if upgrading
your install of Zen Cart helps, in case there is a fix for this in its
code. As I saw no kills of your processes, I can only think the PHP
memory limit is being hit after a length of time. Be sure to let me know
if you still have no luck after the version upgrade.

I have never upgrade zen cart and somehow this is tied to about 4 stores which are not mirrors, but apparently pull from the same data. we change in our main store and the other stores all change according to that at least in products.

There have been many changes/improvements to the download handling process in the 5 years since v1.3.7 was released.

Upgrading would be the best first step.
And, yes, your implementation sounds like the upgrade may get complicated since it apparently affects multiple stores which you've tied together somehow. All will require upgrading, since whichever ones aren't upgraded will start to trigger errors until they are upgraded as well.

ok then, is upgrading difficult then?

Upgrading is only as difficult as the degree to which you've altered the original core code with your customizations and addons.

You can start your reading about upgrading here: https://www.zen-cart.com/tutorials/index.php?article=98 And follow the links to other articles which go into more detail.
It's not something you'll finish in 20 minutes though.