Earlier this week I was playing with my 386 DX40 computer and noticed my memory timings were kind of very loose.

This computer is build on the Biostar 1340 motherboard with soldered AM386 DX40.
http://www.uncreativelabs.de/th99/m/A-B/31865.htm
Currently running 8MB ram (30p):
4x 1MB topless 60ns + 4x 1MB nameless 70ns
Cache: 128kB 15ns, UMC, 5x 32kx8 15ns; had no slower available :p

Other hardware used:
Vtech Laser multi I/o (http://www.uncreativelabs.de/th99/c/U-Z/21378.htm)
Some ET4000 video card
Some ESS Sound drive with IDE for cdrom
Conner 170MB hard drive + CF card, 5,25" and 3,5" fdd
AT PSU with good / stable voltages (can't check for ripple).

Timings for ram were auto configured at 1ws
Timings for cache 1ws and 3-2-2-2

Cache timings allow for ws 0, 1 and 2; 3-2-2-2, 3-1-1-1 and 2-1-1-1
I set them to 0 ws and 2-1-1-1 with no problem: system runs stable on it.
Cachechk returned cache runs 37,5ns.
Shouldn't it run at 25ns at 40MHz? At least my 486 does with a 40MHz bus.

Ram speed is set to 0ws now and cachechk returned it runs at 78,6ns.
When I exchanged the 60ns ram for 70ns ram, the system would hang at loading himem, or himem reports unstable ram.
Setting ram to 1ws solves this error and puts ram to 100ns.
Changed back to 60ns of course and run them at 0ws, 78ns.
The set of 70ns ram I already have in there is fully stable at tightest timings.

The other 70ns ram chips I have are tested on a 486 DX50 and are stable above their rated speed: 4 sets of 4x 1MB 70ns ram running 67ns stable.
I paired 8x 9chip ram, 8x 3chip ram, tried mixing them... Stable on the 486 and unstable on this 386 (both 0ws, tightest timings)

Is there any explanation why these won't run stable at 78ns on my 386?