why do I go from gold 2 to gold 5 based on 1 game?

working as intended, they know who the bad players are and place them based on their ranked

i've beaten challengers and got placed into gold 4

http://boards.na.leagueoflegends.com/en/c/developer-corner/7i1WBYXy-update-on-preseason-ranked

Because of the MMR mismatch in Flex Queue, you got reset to the original starting point (pre placements). But remember this is only Preseason, the rank you accomplish now WILL BE RESET AGAIN! The rank you get in preseason does only influnece the placement in Season 7 to a SMALL extend! (only your starting point in season 7 is affected by Preseason ranks) Sidenote: 1-2 Tier drops are completely normal in preseason.

I.E.: You were Plat 5 at the end of the season, were placed high Silver after placements, now got placed low Silver. If you now climb to Plat again, you will then only be placed in low Gold at the start of Season 7!

So dont worry to much about the current placement!