I answered your email earlier. I wanted to just boil down the suggestions here, so that we'd have the conversation on the forum. I probably won't be able to check back for several days--I'll be on the road.
Here are my top ideas about your dilemma:
1) The Line6 people probably don't realize the intensity of the MIDI you're throwing via that USB piece. Although USB can be a little sketchier than Firewire on a laptop, the bottom line is that the Toneport is probably not made for the intensity of MIDI/Audio that you're bringing with the Zendrum.
So, my recommendation would be to perhaps check out the PCMCIA option (since you could easily return that) and see if that solves anything. If that doesn't work, then I'd check out Firewire options from Echo. They've got EXTREMELY fast drivers. Ultimately, it's the quality of the drivers (ASIO in this case) as they relate to the piece of hardware and OS, much more so than any type of motherboard/hardware/chipset compatibility. Put as politely as I can muster--they fed you a line of baloney about compatibility. It's their drivers, not the chipset.
2) You could look into a Receptor. Frankly, I'm cold on that choice. You're paying a LOT of cash for old/slow technology. For the same cash, you can rig a killer 4-space Rackmount PC, blazing fast, with actually a lot more redundancy and faultproofing capability than you'd have with the Receptor. No slight to the piece...it's just that music software technology continues to explode, and by this time next year, most sampling apps will not run on the Receptor you purchase today. You have to be smart and disciplined about it, but I've been performing with a Rackmount PC since 1999, and I've seen a lot of other solutions come and go, while mine has never let me down.
Your problems with the laptop, I would say with almost 99% certainty, are related to the MIDI/audio interface. If you're going to perform with any type of computer, the MIDI/audio hardware has to be rock solid with great drivers. Echo is only one of many ways to go, but it's the one reasonably priced brand I can give a hearty recommendation for. Personally, even with a laptop, I'd opt for something that screws into a rack, and is road-ready. That is going to be far more secure onstage than wiring up a bunch of tabletop stuff to a laptop...which is a rather precarious stage situation. I could see you setting up a 4-space or so rack with a power conditioner and a couple of spaces left open, so that you could actually pack your laptop in a bag inside, and just pop the lids, unpack the laptop, bang in the firewire cable, and off you rock.
I'll check back on Monday...good luck.