The other possible issue is old, bad, worn out copper. Either at your premise or at the CO. I've dealt with this before quite a few times. Not sure what you can get them to do to have the copper upgraded, but it'll probably take a few more calls and complaints. 900 Kbps upload is completely unacceptable. I would complain about this till the cows come home
Even though a full T1 is technically supposed to be 1.544 Mbps down/up- with overhead you're more likely to get something in the range of 1.4 (give or take a few bits). But to have it all the way down to less than 1 meg?? I wouldn't accept that.
Assuming it's not issues with your equipment, bad copper, or just problems with the carrier's network- have you factored in that it could be because it's an IA T1, rather than a stand-alone data T1? As I'm sure you already know, bandwidth is used every time a phone call is made. This would of course explain the inconsistency. And perhaps when a call is ended for some reason the bandwidth is not reallocating properly. Sounds far fetched but just wanted to throw that out there.
Where are you in your contract term? Did it just start?