I’ve noticed that in the times that I get ahead during a cash game, my “winner’s high” makes me vulnerable and I can lose quite a bit of my stack this way. Is it rude or not if I decide to cash in after a winning streak or so, then come back with just the buy-in amount?
Yes it's rude, or rather many players will consider it rude. However you're playing I assuming to make money and not friends, so if you want to cash out and buy-in again that's your business.
However many card rooms will have policies that forbid this, as effectively you're taking cash off the table, which they tend to not like. The wording of these type of policies tend to be something like a player cannot remove chips or cash from the table while still actively playing. If your card room doesn't like players removing chips from play, then a way around this is to get up, take a break and request a new table.
As a side point, as mentioned by @Paparazzi, having a big stack is a great situation to be in, and the crux of your problem sounds like you are entering to many pots and not playing big stack poker correctly.
Many cardrooms have time limits in which you would need to buy in with the same amount you left the table with.
For example; if you leave the 1/2NLH game at 8pm with $1000. You can't buy in for less until 9pm.
Although, this doesn't mean you can't buy in to other games or other stakes of the same game as usually the rule applies to the same stake of a particular game.
As for the rude part; Always remember you are legally taking peoples money in front of them. You aren't there to make friends.
If you want to do this, you have to stop playing at the table. I mean leave the table and come back again. Some casinos wont even allow you to come back in a short time. Some others have people waiting for a spot on a table. So in case you leave, you are not guaranteed a seat later on. Here in Berlin where I live you always have a waiting list to join a cashgame table. In Vegas for sure it is different, but often enough you have to wait a long time after you cashed out. Some people hate it if you do so. I sometimes use to do this online, to net get sucked out badly. On the other hand you have many advantages of beeing big stacked. Anyway, why would you cash out? Do you probably play too high stakes for you bankroll? As a regular player you should be able to suffer a loss of 2 buyins or 3. If you are scared of losing money, poker is probably not for you.