If you're running DOS VESA games from Windows then it's probably your Windows screensaver or some other Windows program trying to take control of your video card while it's in a VESA mode. This usually causes the type of problems you describe.

But i disable the screen saver and i just found out, when I change screen resolution to 640 or 800 this happen to windows too. Also some little sound come out from monitor.....

Could be your monitor going bad, or maybe the video card is trying to use some refresh rate timings that are hard on the monitor..

Are you talking about changing resolutions in DOS or in Windows?

in windows, im now using 1024 16bit color. the sound like some continous electronic siren

You may be using too high or low of a refresh rate for your monitor...

hmm i didnt change anything, it suddenly happen and, err..where to change refresh rate? Graphics Hardware Acceleration?

sorry i didnt install full driver option. i installed it just now and try changing refresh rate to BIOS default, 60,70 or 75 does not work, only 85 the highest rate works (with 640 and 800 not work)
so..maybe my problem is the video card and monitor not compatiple on low resolution(anymore?)? What is the resolution of the PC start screen(whats that call?BIOS screen?)

85Hz is very high, especially if you have a 17" or smaller monitor.

Probably 70 or so. What monitor driver and what video card driver do you have installed? Make sure your monitor driver is at least Plug N Play and not "Default Monitor". If you can find one specifically for your monitor then that would be even better.

Your advanced video card driver options may allow you to override the refresh rate as well.

17" plug N play monitor with PnP Monitor driver(Microsoft).
Video card: Sis 620
Chip type: 620 rev 2A
DAC type: Internal
Memory: 8mb
Feature: DirectDraw 1.0
Software Ver: 4.0

These is the result of my refresh rate test with the 'Optimal' setting: (can / cannot)
1280x1024: 60Hz can but out of screen fit
1024x768: 70,75,85/ 60 sometimes can, screen out of fit
800x600: 85/ 56,60,72,75
640x480: 60,72,75,85 all cannot (Adapter Default also)
I see flicker on every each resolution

I'm curious how the windows setting will effect the "PC starting screen"? Maybe i should change the BIOS setting but i dont know how?

I've learn so much today, thousand thanks to you 😀 I'll try to find more suspicious point

That's a good point. The startup screen's refresh rate is probably hard-wired (fixed), probably to 60 or 70Hz. If your monitor has on-screen controls then it might be able to tell you what refresh rate you're currently running at (look for a Hz value).

Also, you probably won't find any BIOS settings that control refresh rate. Usually the software (e.g. Windows) is in charge of telling the video card what refresh rate to use, and when Windows isn't in charge then it usually picks a default value (usually in the 56-72Hz range, depending on the resolution).

Do you know what brand and model your monitor is? You can usually find out by looking for a label on the back. It may also be helpful to know the year it was made (which may also appear on the back of the monitor)

Good luck!

Monitor refresh rate H: 68.7Hz V: 85.2Hz (at 1024x768 16bit)
Brand: DMC (Made in Korea)
Model: IM767A
Manufacture Date: Apr 2001

The problem is getting serious, I should let it rest awile. Have a nice day:)

I'm not sure what to say, except try to use the lowest refresh rate that you can for all modes and see if it helps.

You might also try installing the newest drivers for your video chip. I found these on SiS's web site: http://www.sis.com/download/agreement.php?id=38008

Its all fine these 2 days, I still dont know how those being fixed... anyhow thx for your help, I know which to focus if I meet theses problem. And the driver u gave me is same as my current one.