Do monks have big ups and downs in their life? Buddhist and non Buddhist people have a lot of ups and downs in their lifes, doubts, fears, anxiety, regrets etc... we understand that as impermanence.

Do monks experience similar things of aftet joining the Sangha their lifes are pretty much "peaceful and flat"?
Take a look at Ajahn Lee's autobiography, page 16 and onward.

His practice slackens, he grows disillusioned with being a monk, and he begins an obsessive debate within himself about whether to disrobe or not:


  But looking at the state of my meditation, I could see that my practice had
  grown slack. I was becoming more and more interested in worldly matters. ...
  
  One day I went up to a hollow space at the top of the chedi and sat in
  meditation. The theme of my meditation was, 'Should I stay or should I
  disrobe?' Something inside me said, 'I'd rather disrobe.'
  
  ... 'The people in the Great Metropolis aren't deva-sons or deva-daughters or
  anything. They're people and I'm a person, so why can't I make myself be like
  them?'
  
  I questioned myself back and forth like this for several days running until I
  finally decided to call a halt. If I was going to disrobe, I'd have to make
  preparations. Other people, before disrobing, got prepared by having clothes
  made and so forth, but I was going to do it differently. I was going to leave
  the monkhood in my mind first to see what it would be like.
  
  So late in the quiet of a moonlit night, I climbed up to sit inside the chedi
  and asked myself, 'If I disrobe, what will I do?' I came up with the following
  story.
  
  
    If I disrobe, I'll have to apply for a job as a clerk in the Phen Phaag Snuff and
    Stomach Medicine Company. I had a friend who had disrobed and gotten a job
    there, earning 20 baht a month, so it made sense for me to apply for a job there too.
    I'd set my mind on being honest and hard-working so that my employer would be
    satisfied with my work. I was determined that wherever I lived, I'd have to act in
    such a way that the people I lived with would think highly of me.
    
    As it turned out, the drug company finally hired me at 20 baht a month, the
    same salary as my friend. I made up my mind to budget my salary so as to have
    money left over at the end of each month, so I rented a room in the flats owned by
    Phraya Phakdi in the PratuuNam (Watergate) section of town. The rent was four
    baht a month. Water, electricity, clothing, and food would add up to another eleven
    baht, leaving me with an extra five baht at the end of each month. ...
  


I won't paste it all, as this goes on and on for several pages in insane detail as he imagined what might happen if he disrobed. I think the amount of detail is indicative of how much he wanted to disrobe right then and return to lay life.

But he concludes:


  Finally I decided to call a halt. My wife wasn't what I had hoped for, my earnings
  weren't what I had hoped for, my children weren't what I had hoped for, so I left my
  wife, was reordained and returned to the contemplative life.