clock slows down under load

[Marty <martyb@ix.netcom.com>]

I am running sarge and debian linux-2.6.8 on a 486 which appears to be 
losing "tick interrupts."  When the system is not under load the clock 
drifts about 8 seconds per day.  The drift increases to about 6 seconds 
 per hour under 100% load, and seems to vary linearly with load.

I see nothing in the logs indicating a problem, except possibly for 
"kernel: spurious 8259A interrupt: IRQ7" which appears only when the 
system is booting, along with a subsequent "kernel: lp0: using parport0 
(polling)."  (The partport is hardwired to IRQ7, and I can almost rule 
out IRQ conflicts.)

Here are some of my questions:

1) Is it possible that the 1000hz tick rate is too high for the 100Mhz 
486?  Doesn't the tick timer use the highest priority interrupt, and/or 
an NMI, and if not why not?

2) Are too many spurious interrupts the problem?  How do I investigate 
without any test equipment to probe the hardware, or hacking the kernel?

3) Is there a simple way to reduce the tick timer as a test (other than 
downgrading the kernel?)