Quoted from ChipScott:

I remember when I had my premium, and noticed issues with the sarcophagus lock mech registering balls... Very frustrating.
But there is a fix.
I recommend doing a forum search on this issue, which will bring about several posts, and also a separate maiden issues thread, on the matter and you will be able to read through those issues and I'm sure they will sound familiar to the experience you're having.
In brief, while there's little things you can do, the ultimate durable fix is going to be loosening the sarcophagus lock mechanism (such that you can access the switches underneath the playfield at the mech) and changing out the two stock micro switches in there to micro roller ball switches (easily obtainable at Marco).
Out of interest, utilizing micro roller ball switches at the sarcophagus mech was how the game was originally designed, but at production they opted on standard stock micro switches at that mech, most likely because it saved them a couple hundred bucks at the end of the production line.
While not occurring on everyone's game, the gist is that those standard micro switches get bent after a period of time in that mech and ultimately don't reliably register. I think it must have to do with something in the dynamics of the moving mechanism, it's resting angle and the weight and motion of the steel balls constantly rolling over them.
Once you change out to the roller ball micro switches, my guess is that the sarcophagus mech problems you have been experiencing will cease to exist.
Btw, I also don't have the power switch you speak of inside my current IMDN Pro.
As far as the start button, are you saying that it doesn't light up, but starts a game once you press it? If it is the ladder, take a look on the inside and make sure that the previous owner didn't splice into power from the wires at the start button. I know there are a few mods out there that have been utilizing that source (for example, that very cool mod from Yoyokopter, the T-Rex alive mod, utilizes power from that area).
Hope the above helps.