The Accufab doesn't use a diaphragm. it uses a spring loaded steel ball that seats (poorly) into a beveled aluminum seat. That was the 'go to' FPR' for quite a while it seems, but thinks to the powerlogger they've pretty much got an established record of having very choppy fuel pressures and rapid bleed down, where the Kirbin unit is much smoother. (there's a chance I think worse of them than they really are as I've never presonaly ran one. I've just taken them apart trying to figure out why the choppy ness and rapid bleed down) I've seen it repeated often, but it might be that I only hear about the one's that won't act right, even brand new. (plus you have to have a powerlogger and a transducer to track it. Most people don't have a data logging capability.
Now on to your specifics... if you're return line is restricted, that will cause higher pressure than you want. To have a spike that's greater than 60 doesn't tell me much because I don't know what your boost level it. 60 is normal if you have a 45psi base and 15psi of boost.
You've verified the vent and return aren't clogged. Have you investigated the signal line yet? Years ago I helped a guy figure out a very odd problem similar to yours (he just wasn't blowing diaphragms). Turns out there was a rust flake in the line acting as a check valve. When he'd shut off, the pressure would hang out in WOT pressure range, blow black smoke and go pig rich.
Once he serviced the signal line and make it transmit accurate manifold signal, all was fine.
