1) Use the latest miro v2.0 drivers. 

2) Edit CARD0020.INI file (\windows\miro\timing\CARD0020.INI) 

3) Find the resolution you want to modify (for example [1152*864*16@75Hz.VESA]) 

4) Add ´mclk=xxxxxx´ to this section. xxxxxx is the value which will determine the clock frequency 

5) You should start from 100000 and then go higher with small steps (like 100000->105000 etc) 
    For example Windows 98 won't start with 140000 and is unstable at 135000. 

6) When the system is unstable (distortion, computer crashing etc), you should lower the value 

7) If the Windows 9x won't start at all etc, just enter the dos mode at boot (press F8 and then choose command prompt only) and edit 
CARD0020.INI and lower the value. 

Here's an example: 

[1152*864*16@75Hz.VESA] 
mclk=130000 
pll=108000 

With this mclk setting my system (P188/64MB/V1000L-P) is stable.