Checking your powersupply might also be an idea.
Mine A1200 with apollo1240 also boots with a black screen sometimes and it happend more and more often until i switched the PSU, now it boots most of the times again.
The voltage is still a little low if i check it at the floppy power connector but ok if i check it at the main power connector so i guess thats the problem when it doesn't boot.
