I'm starting to hate the m2em
After several hours of experiments I can say - Fixing the network will be a pain in the ass.
If I limit the Network to 58 FPS (which will cause slight audio stutter) the network stays perfectly in sync (and flicker free) in Daytona, Indy500, Motoraid and STCC - But the timer goes absolutely wild in Sega Rally.
On the other hand, if I "unchain" the Network FPS - At about 240 FPS (that means 4 network cycles per video frame) Sega Rally works fine, but Daytona is flickering like ---steaming pile of meadow muffin---, and the others show a network error pretty quick.
Guess it will be back to MAME for a while - In MAME the games seem to work better, no flickering and no link problems in Indy500 or STCC, but Sega Rally is not working correctly either.
I really need to figure out the network stuff...
Most likely the network needs to be "in sync" on scanline 383 (the VBLANK-IRQ is triggered at this point) as most games start to write the network data on scanline 384 and read the network data on scanlines 385 to 387.