Fluctuation of high speed internet.

10 days ago we received a free upgrade from 300 speed to 500 speed,after which starting notice that Ps 4 games were crashing very few moments. called cox told them about the problem. they sent a tech within 2 days to look at it. Fluctuation, is one moment 250 then drops to 86 and lower at times. Tech replace old cable wires from street to house, and then exchange the modem.  Now, even worst than ever can't even do a run in destiny 2 without it crashing, every 3 mins.  again i did testing on PS 4 directly hooked-up too, and received 195/6  per min and retest shows 300 30, then back down to 122/4. and again crashing: cant even hold a game up.  Ok called cox again and in two days again will be here. Also hooked up several computers directly same thing happening. speed drop/ fluctuations. Verizon looking better every moment. any suggestion for the tech, on Monday?