Try to set the safest values in BIOS setup.

If still not working your motherboard is likely to blame.

Maybe something wrong with EMS/XMS memory and dos4gw restarts on some fault?
Just an idea as i have no proposal to how to check or confirm it... (Maybe replace dos4gw with dos32a and see what happens?)

@mpe: I have tried default BIOS setting with no result. If it is the MB do you have any suggestions for testing it? Some specific software maybe?

@GigAHerZ: I'll have to try your suggestion. I'm getting a "fatal error (1012)" when using dos4gw without ext. memory (and the reboot with)

Thank you

You shouldn't need to replace the extender. Use whatever DOOM/Duke ships with. Also the DOOM does need neither XMS nor EMS so try to boot without HIMEM/EMM if you use one to avoid any rare compatibility issues.

Try to disable the L2 cache if your motherboard has one. Check the board for any damage and for obviously damaged components/capacitors. Check voltage levels. Failed capacitors is a very common problem of old boards affecting stability.

Every time I have had this happen to me it was the power supply. Each time the power supply seemed to work as normal so it took a while to find the problem. Try another power supply if you have one, don't change anything else, run doom and see if it then works without resetting. What motherboard/processor are you using.

Is the voltage set correctly? Did you try a different CPU? The games you mention all switch the CPU into 32-bit protected mode. So I would start trying to rule out a bad CPU first.

@mpe: I have tried disabling the different caches with no luck. I'll have to check the board thoroughly if everything else fails.

@AvalonH: I was hoping your suggestion would work but unfortunently i did not.

@derSammler: All the jumpers are set correct on the board but I found out I'm missing a daughter board for voltage regulation. I can't find a daughter board or diagrams for one anywhere. So now I'm getting a DX2 66MHz 5v CPU to check for both CPU and voltage.

Thank you for your suggestions, I'll return when I have testet the new CPU.

Motherboard: FIC 486-VIP-IO2
CPU: am486DX4 100MHz
(New CPU: 486DX2 66MHz)

mpe wrote on 2020-02-25, 13:46:

Also the DOOM does need neither XMS nor EMS so try to boot without HIMEM/EMM if you use one to avoid any rare compatibility issues.

Yeah, sorry about wrong terminology. What i really meant is just any memory beyond 1MB.

[SOLVED]

It was the voltage, I was using a 3 volts CPU without the voltage regulator daughterboard. I have now gotten a hold on a DX2 5 volts CPU and it works perfect.
I'll be getting a daughterboard in a couple of weeks and I will be testing it with the 3 volts CPU.

Thank you for all your help 😀