ThinkPad 240 does not find HDD frequently

[mr.boy]

Hello.
I have problem with booting old ThinkPad 240. It often does not find HDD during booting and write "No operating system found". BIOS does not show HDD too!
But sometimes BIOS shows HDD and everything works fine (I have succesfully installed Win 98 etc.).
I have tried 40GB disk and the same problem occurs.
Some times it boots up with no problem and some times it does not recognize HDD...
Can be reason of this problem e.g. worn-out connector of hdd?
Thank you for any ideas.