Hello,

Lately I have noticed some instability with the motherboard which Checkit reporting 4.77Mhz on an 8088 but different
character video memory speeds when the machine is turned on. At start it reports 640 characters, but a few hours later
780 characters. Another thing is that the landmark diagnostic utility (not the rom) reports a faulty 8253 timer, and hangs
the system, so I decided to check things out. When I got this machine, I de-soldered the oscillator and replaced it with
another oscillator crystal, then later put the original back. The machine worked after that, but I wanted to check that out because
I thought the soldering of one contact was bad, so I re-soldered it, and then the machine started to do continuous beeps
after 20 seconds.

I have replaced the trimmer capacitor because it fell apart, and tested it with a different
power supply that is known to be working, and the result is the same. Then, I replaced the 8284 chip and replaced
the 8253. After doing that, the ATI VGA Wonder+ card shows sharper lines on the screen when it
tries to detect the monitor. When I configure SW1/SW2 block to correct settings, like no co-processor, floppy,
256 onboard RAM, the motherboard no longer beeps, but it seems that after the memory test, the
LED on the monitor starts to change brightness and some lines appear on the screen. After a second or two,
the video card starts to re-detect the connected monitor. I think it's doing a system reset. Nothing
is shown on the screen and I'm not sure if the Landmark Diagnostic ROM would help, or to get some
equipment.

If I purchase an oscillator, would a cheap 100$ be sufficient for the work to be done, or a logic analyser
would be a better choice?

According to the diagram, it seems like that a gate is failing or some of the LS175 chips, but I'm not entirely sure.
Also, I think there is a frequency problem with the ISA bus, but I don't know where to look at the diagram.

The beeps are loud and clear without any small noises.

Is there a suggestion on what IC I should check?