Curious, Skeleton cycles it's PRNG and does some other things during VSYNC.
Maybe a look at the Solaris & Desert Falcon disassembly might provide some
hints.
My guess (based upon the NTSC docs I've seen) is enabling VSYNC causes the
TIA to output the serration pulses (NTSC lines 4,5 & 6) which are low except
for two pulses in the middle & end of the line.
If this is true, then VSYNC needs to be enabled & disabled during the
horizontal sync pulse period of the line. So, as long as you do the STA
VSYNC immediately after the STA WSYNC (and don't hit RSYNC while VSYNC is
enabled) you should be okay.
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