Post your DOSBox config.

[sdl]

fullscreen=false
fulldouble=false
fullresolution=original
windowresolution=original
output=surface
autolock=true
sensitivity=100
waitonerror=true
priority=higher,normal
mapperfile=mapper-SVN.map
usescancodes=true

[dosbox]

language=
machine=svga_s3
captures=capture
memsize=16

[render]

frameskip=0
aspect=false
scaler=normal

[cpu]

core=auto
cputype=auto
cycles=13000
cycleup=10
cycledown=20

[mixer]

nosound=false
rate=44100
blocksize=1024
prebuffer=25

[midi]

mpu401=intelligent
mididevice=default
midiconfig=

[sblaster]

sbtype=sb16
sbbase=220
irq=7
dma=1
hdma=5
sbmixer=true
oplmode=auto
oplemu=default
oplrate=44100

[gus]

gus=false
gusrate=44100
gusbase=240
gusirq=5
gusdma=3
ultradir=C:\ULTRASND

[speaker]

pcspeaker=true
pcrate=44100
tandy=auto
tandyrate=44100
disney=true

[joystick]

joysticktype=auto
timed=true
autofire=false
swap34=false
buttonwrap=false
circularinput=false
deadzone=10

[serial]

serial1=dummy
serial2=dummy
serial3=disabled
serial4=disabled

[dos]

xms=true
ems=true
umb=true
keyboardlayout=auto

[autoexec]

fullscreen=true
aspect=true

use these

try also scaler=none

remember that dosbox can't use 320x200 resolution, only 320x240...

FulValBot wrote:

remember that dosbox can't use 320x200 resolution, only 320x240...

false. As VGA is doublescanned these resolutions will become 640x400 and 640x480 respectively, and there's even some modern video hardware that still support those very low resolutions (Intel HD comes to mind)

You also shouldn't need to use aspect=true on a CRT either as they already project all resolutions at 4:3 unless manually configured otherwise.

I'd either blame the odd VGA adapter setup or the possible use of a third-party build...

Yes, probably the fault of the DP to VGA adapter. The GTX 1070 does not have an analog out, so those adapters are not passive, and therefore result in "get what you pay for" performance of its DAC. Products like these assume 99% of the use cases are 800x600 / XGA / 720p projectors. The adapter is probably assuming a minimum resolution and has logic to pad whatever comes in less. 320x200 will look tiny masked with 800x600, but it guarantees the output device can show something and prevents one less phone call to product support.

You might be on to something with the DP-VGA adapter. It is just a cheapy eBay adapter with no documentation. Can any recommend a better adapter?