Download a fresh copy of MT4 from the broker and install it in another directory. Allow it to install any new updates, but don't add any of your own software. If the problem persists and the broker doesn't want to help, it doesn't matter how honorable they've been in the past. It's time to change brokers.
I'm hesitant to recommend cTrader. They do prevent broker price tampering, but allow the broker to confiscate profits because of "price issues" and won't assist traders in checking to see if these are true claims or not. If you can get cTrader with a trusted and well-regulated broker, it's probably safer than MT4.