Is this your only Mac? In other words, do you happen to have a functional scsi hard drive with a 68k system installation on it, 7.5 or above? That would be useful for testing purposes. The custom card is likely to be temperamental.

However this may be, as I said some posts ago, I have had two 840av boards that needed washing. I did mine in a dishwasher (empty, no soap, and yes, it works), but search for alternatives if you do not have one. It is a common practice. Replacing the capacitors is a longer-term solution, but that may not be necessary in morder merely to make the machine boot.

Logic boards were washed with water at manufacture, so it does no real harm; you can also do the cleaning with rubbing alcohol, but I have not found this method any more effective than using simple water.