Customs

The 90-day limit applies to most countries in western Europe as a whole, not just one country, so your time in Amsterdam counts, too. I don't have direct experience with overstaying, but my understanding of the Schengen (spelling may be off) rule is that there are stiff fines that can be applied. The limit is 90 days out of 180 days (and going outside the Zone doesn't restart the clock, as I understand it).

So where in europe would I have to go in order for my 90 days to be restarted when I get back?

Your 90 days don't "restart" until 180 days elapse after your initial entry. First of all, "customs" is not who stamps your passport. That occurs at immigration. Customs is a separate line that probably 99% of all visitors from the US bypass, unless you have some specific items to delare . Most countries in Europe are signatories to the Schengen Treaty, which among many other things, eliminated border immigration checks (but not all customs checks). So, your passport gets stamped upon arrival in Germany and won't be stamped again until you leave the Schengen zone. Going outside the Schengen zone stops the progression to 90 days, but doesn't reset it unless you stay outside until 180 days have elapsed since your initial entry (ie, the first time you landed in Germany). The penalties for overstaying the 90 day limit vary from being barred from return, deportation, fines, and even prison, depending on the exact nature of the offence.