I'd be surprised if the 51A would support 5.2.
I found this p510 redbook:
http://www.redbooks.ibm.com/redpapers/pdfs/redp4136.pdf
Which says:
"If you are installing AIX 5L on the server, the following minimum requirements must be met:
AIX 5L for POWER V5.2 with the 5200-09 Technology Level (APAR IY82425), or later.
AIX 5L for POWER V5.3 with the 5300-05 Technology Level (APAR IY82426), or later."
If you consider these may be the minimum requirements for the 510 the the 51A may well need higher levels and not support 5.2 at all.
Make sure you have AIX install media at least at 5.2 TL09 - out of support already so install 5.3 if you can - at TL09 if poss because TL08 goes end of defect support in about a month - and let us know how you get on.
If you still have problems let us know what monitor you use, how it is connected, what settings you are using, Etc.
If poss try installing 6.1 just to see if that works.