Yes, 90°C is a lot and is one possible cause to freezes - it did on my 440bx mobo. If you increased frequencies then 90°C can well be too much, as hot silicon is slower. Or your chips got hotter and you didn't see it: for a current Cpu, it's a matter of two seconds.
Other chips, for instance on the graphics card, can have the same effect. They sometimes display artefacts before freezing, sometimes not. Or the Cpu, the Ram...
The Arctic 7 is great to cool a Cpu silently, but as it blows air more slowly, it makes life harder for the Northbridge. Did you orient it to blow on the Northbridge?
I'm a bit surprised that adding a fan, even small, brings so little improvement. Could you try other means to measure the temperature? You finger first, then if possible a software provided by the mobo's manufacturer. Figures in the Bios are the most reliable BUT don't represent at all working conditions.
Third-party coolers exist for Northbridges. But I would first take any strong blower (like the small strong noisy ones from the PII era) and hold it by hand over the existing cooler. It the temperature doesn't improve clearly, it would cast doubt on the measurement. Though the chip-to-cooler contact could be bad.