Running a 1.1GHz/100MHz FSB Coppermine together with a SPC370 slotket on a BH6 in my PIII rig. My guess is that any Coppermine capable slotket will work just fine, the SPC370 is certainly nothing special.
You don't state where you live, but got a tip from another member a week ago; there's a a German dealer on Ebay with a surplus of MSI MS-6905 slotkets right now, selling at very reasonable prices. One of those would probably be a very safe bet. Picked up a few myself for later use.
You can do an equivalent to the CPU mod on a slotket, if you want to use a Tualeron. It's not all that complicated if you can handle a soldering iron. The BH6 can supply the required voltages without modification.
On the GPU note, the GF Ti4200 in my PIII rig works perfectly on 100 MHz AGP. Accidently set the divider to 1/1, took at least three months before I noticed, and as far as I can remember, nothing misbehaved during that time. Sure, might be differences between individual cards, but I hardly think your GPU is the problem. If you ask me, the GF4 series is as good as it gets when it comes to handling overclocked AGP buses.