r/NikonF3

i got a good deal on a Nikon F3HP, all speeds seem to work fine or at least sound like they are within spec. the speeds below 60 are strange though, from what i was able to test with a spot watch on my phone and my eyes and ears i get about 9s to 2min 58s of exposure time in the 8s exposure mode. these remain sporadic even in auto mode.
i tested all of these at a normal shooting shutter count (always above 1) i tested different ASA numbers, exposure compensation values even different apertures (set at the aperture following ring)
im lost here, i do know my way around repairing stuff but this is very strange and not something like a sticky shutter (i tried lightly hitting the camera while it was above its set exposure time to "unstuck" the shutter, sadly that wasn't the issue …)

Does the Nikon F3 allow you to fire the shutter while holding down the DOF preview button? Mine doesn't seems to, the mirror will be stuck halfway