[Blas]

Most of the problems with MSD Ignition Systems could most likely be traced to improper electrical connections. This includes a bad ground or power connection or a cheap ignition switched that breaks the power contact supply to the MSD box's small red wire causing intermittent power surges.
Another box killer is a power spike. If you flip your battery cut-off switch off while the engine is running, you'll get a power spike from the cars power source switching to the alternator from the battery.
(Bet you thought the car would turn off – I think the vast majority of them will not – but that’s another thread)
I believe the MSD instructions suggest grounding the unit as directly as possible to the negative terminal on the battery.
MSD also makes a noise capacitor (PN# 8830) that in addition to insuring clean supply line power to the MSD box, also protects the MSD box from Voltage spokes and current surges.
Mounting the MSD box upside down allows moisture (from rain or temperature swings) to collect in the cover. So if you drive in the rain, or if you do some crazy car washes you run a risk…
The under cap problem is a different issue. Checking the phasing and checking the interface of the rotor and cap contacts is time well spent. MSD does not preset the rotors center contact at the factory. Many a cap has been destroyed from arcing on the center carbon contact.
Another suggestion for the corrosion problems inside the distributor cap could be due to poor grounding of the aluminum cylinder heads. This can cause excessive heat in the cap leading to condensation and corrosion. Where you live and how you drive your car also make a difference, if it rarely gets up to temperature and stays there a while there is no chance to burn off any accumulated moisture under the cap.
Someone once wrote that the height of their rotors end contact in relation to the dist. caps 8 contacts was too big, so he shimmed up his rotor with washers to lessen the gap. I would tend think that maybe some mis-matched parts were actually installed in this particular case.