make sure the IC's are seated?

That sounds like a 5v issue. Check the uppermost right connector on the solenoid board.

Have you checked the tilt ball, pendulum and tilt switches? I had a pendulum on one of my games get stuck after moving it up the stairs once. Was pulling my hair for about an hour until I lifted the playfield and saw it.

Start with the fuses on the rectifier, test them only after popping each one out of the fuse clip. Also measure the TP's on the rectifier

How do the rectifier TP's match up to the MPU test points?

Move it back to it's original spot and see if it works. You may have a machine that susceptible to the Moon's lunar electro gravitational waves. Ever see a older Bally or Stern solid state that all the displays seem to be strobing? Well, there you go.

Moving a pin across the room causes more failures than everything else combined!

I had a somewhat similar issue once that turned out to be an intermittent short caused by a loose screw hanging off a grounding braid...very hard to find.

Did you take the backbox off? If so, check the square Molex connectors in the head (6 pin, I think) and make sure the wire colors match. The connectors aren't keyed so it's easy to mix them up.

There should be a lone braided grounding wire running between the cabinet and backbox--have you made sure that it was attached?

I would assume so. Can't say I've tried it intentionally, though.

That braided strap is the 'safety' ground that runs all over grounding metal parts that might could shock (even kill if they're wet and barefoot) someone. The circuit boards carry their own grounds through the connectors. The strap is not necessary for the game to boot.

Does it do anything when you turn it on? Any flashes on the MPU or beeps from the MPU?

You mentioned that you did work on the sound board. Try unplugging the ribbon and power cables on the sound board and try again. A bad sound board will keep the game from booting. Also, because there are two ribbon cables make sure you are not off a row or skipping any pins. Make sure the cables are attached in the right order. There should be 2 open (unused) pins on the left side of the sound board and MPU.