I'd suggest clearing your browser cookies first to see if that helps with the site loading issues. Also might be worth running a quick check of your data connection (http://speedtest.net/).

The upgrade ineligible/eligible issue might be tied to skymiles numbers not being properly pulled into the system until during the final booking process. I've noticed that issue when searching prices, but had it resolve as I start the booking process (where you start entering your name and whatnot, but before you hit the pay button). My hunch, dl.com is getting mixed signals between "upgrade requested" but not properly passing off the SM#/eligibility during the first stage of shopping for your flights. Once you start actually entering your info (which pulls data from the servers behind the scenes to fill in the blanks) I've always had the ineligible switch to upgrade eligible. Mildly annoying yes, but at least for me (FO) I've always seen it resolve properly before I hit 'submit $$$'.

Hope this helps!