Slow Gigabit service

Hi all , having issue with speed.  Been working fine for over a year now getting 900+ at the modem, 600-700 wireless.  Starting June 1st speed is gone.....less than 100 at modem and 20 or less wireless. No changes on my end.  Called tech support and reset everything.  Will work well again for an hour or so and down again.  Tech came out and thought he fixed it, replaced cable and fix grounding issues and looked to be working fine again.  Couple hours later 100mor less again????   Am I being throttled??  Any ideas appreciated