Uhh..
Avg/s = 9.9MB
Not good..
Yep, i think this is definitly your problem. Let me explain:
If you are playing a stereo track @ 44.1/16bit then you will average about 0.2 MBytes/s read throughput. Multiply that by 15 and you and you get roughly 3MBytes/s. Ok you think, i should be able to mix 15 tracks no worries with 9.9MBytes/s avg.
But the problem is, when you hit play in your DAW. Your hard drive is sitting there chilling out, having a smoke, coffee, whatever, when all of a sudden chopper boom! It's in audio central hanging out with windows overhead. Or in other words there's a lot of extra info to deal with at first and it will queue up and cause an initial spike of approx 10-15MBytes/s for 15 tracks when you first hit play. So of course it, can't deal with that when there's only 9.9 to play with and causes delay.
So you'll probably notice that it works fine after this initial spike, as the throughput smooths out and goes back down to around 3MBytes/s... but then,
that's without considering extra overhead from plugins/background apps/windows services and such that will cause more spikes and random dropouts for you.
As for an external sound card on USB 1. This is ok, as long as it only has a couple of inputs on it. You just don't want to hook your storage to it.
Thats the end of my theories.