Strange. Using QT Mobile hotspot (for both WiFi semi-access point, and as an USB one), I never had any problems. Like, at all.

Considering, that hotspot mode is de-facto ad-hoc, maybe something is affecting your ad-hoc cinnections, too? Have you tried estabilishing one (via vanilla Maemo's network settings) and check its stabilit?

Not sure if it is an option for your case, but you could use Bluetooth PAN network sharing - it uses much less battery power, and is not less safe than WEP-only ad-hoc hotspot...

/Estel