Is comcast throttling download speed from certain IP#s?

Has anyone seen comcast biz internet greatly slowing down transfer speeds between your site and specific other sites? (Likely by IP#)

I have comcast biz internet at my main location, and have servers at two remote locations (one using AT&T, one using Centurylink). I transfer backups and other files between them. I've noticed that "download" file transfers from my two remote sites into my comcast site are slowed waaaay down, like 300K bytes/sec (i.e. about 2-3M bits/sec).  This is on a 750Mbit/s comcast service with static IPs (i.e. rated for approx. 75-100M bytes/s down), so 2Mb is like 0.3% of the rated speed.  When I route that exact same traffic over a VPN (att -> vpn -> comcast), I get much faster speeds, like ~150Mb/s (which is in line with the speed for the VPN service that I see to other sites). The AT&T service is symmetrical fiber at 500/500Mb/s, so 2Mb/s is just insanely slow.  Transfers in the other direction, "uploading" from comcast to my remote sites, goes as expected (35Mbit/s comcast service and I see just about that, ~4.5MByte/s).  When I transfer from sites that aren't my remote servers, I see the expected speeds (i.e. the max upload speed of the remote site, say, ~100Mb/s; no throttling). Speedtest sites give me the expected speeds of like ~900Mb/s down & ~42Mb/s up. (Clearly comcast is not throttling speedtest sites!)

Data transferring from my at&t 500Mb/s-up to my comcast 750Mb/s-down sites should flow at something near the 500Mb/s rate; even 100-200Mb/s would feel ok-ish. But 2Mb/s?!?

And the only difference between turtle-ish 2Mb/s and normal, zippy speed is the ip#. (And the fact that I do a lot of transfers between them on a daily basis.)

I thought it might be the remote ends doing the throttling, but that would have to be BOTH at&t and centurylink doing the same thing to my comcast ip# -- whereas the Occam's Razor simplest answer suggests it's comcast that's doing the slow down to ip#s it puts on some list. (Perhaps based on noticing traffic between them? dunno)

Anyone else seen this?

And, more importantly, any ideas how to get my ip#s unthrottled? :)

Thanks for any insights!