Help! I have an HP PSC 750 Printer/Copier/Scanner it is not printing correctly.?

What do I do? It will print the first 4 pages okay. After that there are random blank blocks all over the paper! I did an alignment thing, clean the ink heads, it is still new (2 months old) ink cartidges. What am I doing wrong? What should I try to do to fix this problem!!?!?