Weird lock or bug maybe?

Hi All,

I got 2 very simple tables, one hold few values and got a unique field
to be used as identifier and the other table got a reference to that
table (foreign key).
At my last testing before the problem I inserted to each one of the
tables about 1 million rows, for the process of inserting new
information I begin a transaction, delete all the previous information,
insert the new information and commit the transaction.
So far it never gave me any problems (though its the first time that I
let a foreign key to exist) but now from some reason it "locks" a
specific row from being deleted.... for example:
If ill query: DELETE FROM table1;  it will just get stuck...
If ill try: DELETE FROM table1 WHERE table1_id=1523; it will work in
most cases but for some rows it will just get stuck!
Anyone know anything about this weird problem?
By the way when I restarted the DB server I was able to delete the
current row which stucked the process but then I got stuck at some other
row at the table....

Thanks in advance,
Ben-Nes Yonatan