I misinterpreted the bios notes - p3-450 is the highest cpu on this revision, and it requires P08

I’ve since updated to P14, being the last version released.

The issue remains.

I can use a USB mouse for windows, but that doesn’t help for DOS. I’m currently using the Celeron until I can find some help with the issue.

PS/2 from that era typically powered the peripherals from the 5V rail. Being that the 5V rail is also used for powering the CPU, I'd suggest to check either your motherboard's capacitors or your PSU's capacitors... or alternatively, just post some good photos of them so we can see. Bad caps can cause all sorts of issues... and hardware from this era often has them at this point.
If you have a spare known-good known brand (and preferably newer) PSU, you can try that too.

That's certainly a new avenue to investigate. I didn't even consider the PSU might be just enough for what the system shipped with.
Not seen anything untoward on the board or CPU, I had a look before assembling (not an in depth one) - as everything up to now has been working fine, I assumed it was OK.
Machine is away for the evening, but I'll check tomorrow. I don't' have another PSU to play with, sadly.

I took a quick look at the PSU, and yeah... it's a 90w PSU with a max 12a on the 5v rail.

The tech specs for the motherboard even explicitly state: " The standard SFX 90 W power supply is not sufficient for the RC440BX motherboard".
Serves me right for reading up on the chipset and BIOS versions, and neglecting to notice the "I'm here too" PSU. I'm surprised nothing else is struggling as yet.

I'll have to keep the Celeron until I can get a better PSU, or at least borrow one to test.

If anyone else met the same issue and can advise how they resolved it, I'll take it on board.
On the plus side, I've been saying "That's OK" for the onboard ESS. The Yamaha card works just fine, and man, that OPL3 hits me right in the nostalgia.

Kalador wrote on 2025-02-11, 02:42:

I took a quick look at the PSU, and yeah... it's a 90w PSU with a max 12a on the 5v rail.

That's not too shabby for this system, even if the mobo manual does say that a 90W PSU may not be enough.
12A on the 5V rail translates to 60 Watts of power. The Celeron 400 is a 23W TDP chip... so only uses about 4-5 Amps from the 5V rail. And the P3 450 isn't that much more power-hungry @ ~34 Watts... or 7 Amps from the 5V rail. Now, the difference from the FSB going from 66 to 100 MHZ could be pushing the NB to consume another 5W extra, or 1A more. With an HDD and the RAM, add another 5W for a good measure, bringing the total to maybe 8-9 Amps from the 5V rail. So with that said, I think it may be down to either the caps in the PSU or on the motherboard... thus, I suggest to post some pictures of your board and PSU so we could see. Otherwise, with the PSU rated at 90W, at least it seems like it may be a decent brand and not a cheapo that greatly overstates its power capabilities.

BTW, before chucking down the problem to PSU or mobo cap issues, do you have another PS/2 mouse on hand? If so, it may be worth trying that too. From what I remember, the early MS IntelliMouse were quite sensitive to electrical noise, especially the 1st gen optical ones. I remember these jumping all over the place with noisier PSUs.

So, we can out this to rest now.
I got a new PSU, but it didn't make a difference, which is not too bad as it didn't fit the case any way (Right size, wrong orientation).
In checking the BIOS, I noticed the last update didn't actually take, and I was still on P08.
Turns out P08 was the last Gateway release, and later versions weren't matching the OEM so weren't actually installing.
I also noticed, on further reading that this specific issue was alluded to in the release notes for P09 specifically, just one release beyond the final Gateway OEM revision.

I moved a jumper to put the board into BIOS recovery mode, and "recovered" from P14 (The final Intel revision for the board). This worked fine, and the issue is now resolved. The Mouse is detected on boot with the 100 FSB.

Guess I've got a PSU, a Celeron and some RAM to float on ebay. The only thing I don't like is that the splash screen on boot is now a generic "built by Intel", but I can change that when It annoys me too much.

Thanks all for the advice, I'm still running the stock PSU, as it fits, and seems OK so far, and I'm still using the Intellimouse, so all hunky dory here.

Thanks for coming back to share your results and glad to hear you got that solved! 😀