Vim -- drawing errors, strange artifacts, and improper sizing - vim

Question

When I use vim in bash with the standard MacOS terminal I get this whacky artifact behavior, especially with JSON files. The wrong line numbers will show, and the wrong lines will show. When I move the cursor around, it will 'uncover' the actual characters in those positions, and result in a crazy mess of curly brackets or line numbers (in the case of a JSON file).
When I run vim in bash with iTerm2, the vim window only takes up a set amount of space, which is often less than half the area of the entire terminal window.
Perhaps there's some issue with my .vimrc file?
" ----- Native Vim Configuration -----
let g:netrw_altv=1
let g:netrw_liststyle=3
"autocmd BufNewFile,BufRead *.json set ft=javascript
filetype plugin indent on
set tabstop=4
set shiftwidth=4
set expandtab
set number
" ----- Vim Plugin Manager -----
if empty(glob('~/.vim/autoload/plug.vim'))
!curl -fLo ~/.vim/autoload/plug.vim --create-dirs
\ https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim
autocmd VimEnter * PlugInstall --sync | source $MYVIMRC
endif
call plug#begin('~/.vim/plugged')
Plug 'morhetz/gruvbox'
Plug 'tpope/vim-fugitive'
Plug 'ctrlpvim/ctrlp.vim'
Plug 'godlygeek/tabular'
Plug 'prettier/vim-prettier', {
\ 'do': 'npm install',
\ 'for': ['javascript', 'css', 'scss', 'json'] }
Plug 'scrooloose/syntastic',
Plug 'mxw/vim-jsx',
Plug 'tpope/vim-surround',
Plug 'mattn/emmet-vim'
call plug#end()
" ----- Vim Plugin Configuration -----
set statusline+=%#warningmsg#
set statusline+=%{SyntasticStatuslineFlag()}
set statusline+=%*
let g:syntastic_always_populate_loc_list = 1
let g:syntastic_auto_loc_list = 1
let g:syntastic_check_on_open = 1
let g:syntastic_check_on_wq = 0
if executable('ag')
" Use Ag over Grep
set grepprg=ag\ --nogroup\ --nocolor
" Use ag in CtrlP for listing files, does respect .gitignore
let g:ctrlp_user_command = 'ag %s -l --nocolor -g ""'
endif
colorscheme gruvbox
let g:gruvbox_termcolors = '256'
let g:gruvbox_contrast_dark = 'medium'
let g:gruvbox_contrast_light = 'soft'
set background=dark
Other potential factors: I am using ssh to connect to a pretty beat-up server.
The graphics card on my machine seems to be somewhat flawed. Periodically my display will flash black and I see polygonal glitches when I am using the graphics card.