How are you paying them? Hopefully through a bank and not upfront.
Sounds like a lot of problems for you, but just because they are slow doesn't mean they aren't good. You say they are doing a great job but then hope they won't so you can say "enough". I don't want to sound rude but do you want it quick or right (of course I would want both! LOL)
As far as working on 5 things at once, that's common. Sometimes material isn't there or has been received unusable for some reason. Would you want them to stop and wait for the usable material so they could finish one thing at a time? Our GC ordered trusses well ahead of time but they were 3 weeks late in arriving. That wasn't GC's fault but now our timeline is a over a month behind schedule.
As far as being slobs, what do you define as a slob? Left over food and trash? Or, pieces of wood cuttings and shavings? We're building on a Scenic Byway, so DH told GC upfront that the job site would be kept clean, still 8 months into our build and DH has to remind GC ever so often. If you are paying your GC as work is completed you can easily tell him there will be a penality for any job cleanup you need to have done in the future. He may quit on you but it sounds like that might be ok.
Good luck!