is the 3com an etherlink III?

By default, the etherlink III wants to squat on IRQ5. (Or IRQ10, which would conflict with any tertiary IDE interface that might be on the card...)

This poses a problem, since the LPT port lives on IRQ7, The COM ports live on IRQs 3 and 4, the video card historically straddles IRQ9, and the PS/2 keyboard controller (and or, the USB controller) lives on 12.

All the other IRQs are "NEVER AVAILABLE" because they are always occupied by system resources. (IRQ0 is the system timer, IRQ1 is the AT keyboard controller, IRQ2 is the cascaded interrupt controller for 16bit IRQs, and is connected with IRQ9, IRQ6 is the floppy disk controller, IRQ8 is the realtime clock, IRQ10 is tertiary IDE controller, IRQ11 is "Maybe free" [if you have more than one USB controller, or an IEEE1394 controller present, it is probably on 11], IRQ12 is PS/2 keyboard controller/USB controller, IRQ13 is numeric coprocessor, IRQ14 and IRQ15 are PCI IDE controllers etc..)

This "NOT ENOUGH IRQS!!" problem was just one of the lovely bits of obscura that came with having a vintage machine, way back when.

The usually suggested solution, is to disable COM2/4, and reclaim IRQ3 in doing so. THEN, use the 3COM setup program to reconfigure the card to use IRQ3. This then keeps it from squatting on IRQ5, and lets you keep your GUS/PAS/SB card functioning.

See this thread for said setup program

Since that is a PCI card though, you'd be better served trying to manually configure it with the PnP bios options in your setup program (assuming your Dell has such a thing...)

The other option, is to disable the LPT port, and reclaim IRQ7, and move the soundcard there. (OR, if it is an IRQ10 collision, disabling the IDE interface on your GUS.)

I suspect IRQ conflict.
GUS occupies two IRQs, and you need to set both as "Legacy ISA" in Setup to prevent them being assigned to PCI devices.

I agree. Most likely 5 or 10.

The 3COM is not an etherlink III, it's a 3C905C-TX-M Etherlink 10/100 XL PCI with hardcoded IRQ 11.
In fact I had already disabled COM1/2 and LPT ports in the BIOS and the GUS was configured on IRQ7 for both IRQs.

Since then, I discovered that there is a Configuration and Diag program in http://vogonsdrivers.com/getfile.php?fileid=914&menustate=0
(I didn't use this driver package for the 3COM installation under Win98SE)

I changed few parameters (IRQ is not modifiable though) thanks to 3C90XCFG.EXE (I mainly disable the BOOT PROM), and now I have no conflict anymore.

Everything is working fine now under both DOS7 and WIN98SE.

I'm even able to use Arachne browser thanks to the DOS packet driver found here http://www.georgpotthast.de/sioux/packet.htm

Thank you for your help.

Glad you were able to sort it out!

Yes, I'm very happy!
I can now remove the GOTEK drive which does not fit very well in the OptiPlex case.
Now I would just like to find a decent FTP client under DOS to transfer files without using WIN98/CuteFTP...

M Brutman has an FTP client for dos.

http://brutmanlabs.org/mTCP/mTCP_FTP.html