Which board(s) were replaced? Have you been through these fixes http://pinside.com/pinball/forum/topic/vids-guide-to-bulletproofing-williams-system-6 ?

Were the drops resetting correctly before?

Check EOS switch for the flipper?

Resetting into attract mode can either be the slam switch closing (check to see if it is bent too close together and making contact inappropriately) or probably more likely the 5V line to the MPU not being solid (depending on which boards you replaced, this could be the main 5V filter cap needing replacement, or the male header on MPU or power supply needing replacement, or female plug on MPU or power supply end needing replacement, or some combination of these).

Ok, so if everything is new except the power supply, then replacing the 5V filter cap is probably your first stop. On my games I just go ahead and replace the 5V/12V header while the board is out.