There are a number of things that can cause that, some depending upon the game and specific boardset, so let's start with:
1) where did you measure the +5.2v, at the power supply or somewhere on the boardset? The latter is better, as long as you are careful, because it gives a more accurate reading of what voltage is reaching the boardset.
2) identifying the game and boardset you have.
3) when did it start malfunctioning, did anything occur around the same time such as moving the game, anything change inside the game, etc.?
4) you reseated the PROMS and color PROMS, but how were the legs (dirty, clean, etc.)? How were the sockets?
5) Any ribbon cables on this board and is there more than one board? If so, connectivity between the boards could be an issue.
6) pictures of the boardset.
Please provide more details and we should be able to assist you further.
Scott C.