gonzo wrote on 2023-03-02, 19:45:
EDIT: am I right (or wrong), that the lower CPU-frequency is caused by a drop of the FSB?
We can't say. There are many ways to effectively slow down a 486 system. Dropping the FSB is possible, but it needs to be done with care. A processor like the 5x86 has a "clock multiplier" that generates the 133/160 MHz internal clock from the 33/40 MHz FSB clock. This clock multiplier circuit can't cope with random changes of the FSB frequency, the FSB frequency needs to be adjusted slowly.
Modern 486 clock generator chips can slowly adjust the FSB clock, and the is used by many late 486 "green" motherboards, when power management considers the board "idle". Try disabling power management in the setup to prevent sudden drops of the FSB.
Other ways to slow down a 486 system, that are commonly used to implement a "de-turbo mode": L2 cache can be disabled or the front-side bus might be blocked (using HOLD) for regular time intervals.