Unusual hardware problems

A few months ago, my computer started freezing up completely for no apparent reason. It happened originally in Microsoft setup programs in Win95, and in Word97 in NT4, and on boot up in Linux.
After several days of trying to solve this, I disabled the CPU internal cache in BIOS. The system no longer crashed, but ran like a 486 (It's a P150 with Gigabyte HX motherboard, Grafixstar 600 and 32MB RAM). I then sent the CPU away for replacement, thinking the cause to be a broken cache. The replacement did the same thing.

The next thing was to UNDER-clock the CPU to a P120, thinking that if going slower made it work then this might be a good test. It does work now, but at P120 speed, and with occasional lockups in:
Carmageddon demo
Duke3D and Blood setup
Doom won't start in Win95

Also, I bought a new modem yesterday. It's one that takes processing power from the CPU, so is about half the price. It works fine on a friends machine, but on mine, when I boot up with it in, the screen stays blank, with not even the monitor power light on. The system does the self check and then stops, from what I can tell from the disk noise.

As it stands, the system is usable, but slower than I'd like, and with some irritating habits. And I can't use my brand new 33.6K modem either, so help would be much appreciated.
Thanks
jhubbardAsked:
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henrieCommented:
Your system improved by lowering your busspeed (66-60MHz). This could identify one of your controllers isn't functioning properly anymore. Also mem acces and other speeds are lowered.
Adding your modem and after that not being possible to boot, certainly says something about your busspeed settings or a load to high on your databus.
You'll have to check your BUS-timings set in your BIOS.
What happens when you only install 16MB of RAM?
Disabling internal cache does influence your secundary cache. What happened when you disabled secundary cache?

What once was a working system and now is unstable, also could point out to a defective power supply (also when we keep in mind modems use several supply lines with different voltage)

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jhubbardAuthor Commented:
I bought a new case about a month ago, so there's a new power supply in it since it first went wrong. I tried disabling the external cache, which didn't make a difference. I also tried a cache checker program, which showed up no faults.
I tried my memory in someone elses machine, and it worked fine. Also, isn't going from 150 to 120 a matter of the clock multiplier, rather than bus speed? I don't know that much about it, but multiplying in my head gives this answer. Could my multiplier chip on the MB be broken, if there is such a thing?
I can't remember if I tried altering the bus speeds, but I'll certainly give it another try.
Thanks
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jhubbardAuthor Commented:
Well, I tried the bus timings and other options (they're the ones that go 222/333 and such, right?), and still no joy.

One thing, I already have a modem (14.4, which is why the new one), and that works fine. The difference between the 2 is that the new one uses the CPU.
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henrieCommented:
You are right about the multiplier, I was thinking you lowered from 133 to 120 MHz, which means the same multiply, but another clockspeed. When I reread your question, I saw that is was 150 to 120MHz, so you have indeed the same clock as before (60MHz).
So when the rest of your system is the same, and lowering your CPU-speed gives more stability, it must indicate a problem with DMA, mem i/o peripherals or an unstable CPU Voltage Regulator Module, all present on your M/B.
I think we've run out of options here and it looks like the only option is replacement of your M/B :(
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jhubbardAuthor Commented:
Thanks Henrie, I'll take it up with my MB supplier.
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