Toilet bubling and backing up...

Hi, first thanks to anyone who has input on this for me I really appreciate it. So I have a four family apartment building that's been roughly converted to townhomes, being each unit has an upstairs and a downstairs and the plumbing and hot water was run as one connecting system from hot water heater to upstairs and downstairs. When I bought it it had new electric and new plumbing. The work was done about 5 years ago from what the pipes say and I'm noticing they didn't do I very good job. The building been vacant for a while and we just got the hot water heater in and for the last week everything has been fine. Until recently...i first noticed this when we hooked up the upstairs shower and ran it and it wouldn't drain. After running some hot water for a while and flushing out some of the construction debris it drains good, however, sometimes it overflows the toilet downstairs directly below it. Now today when using the downstairs sink which is directly across from the toilet about 3 feet the sink drained fine but the toilet overflowed. And then it wont go down, sits there and eventually bubbles and its good for a few flushes. Also today when hosing down the basement overflowed water I noticed the main sewer drain where everything is connected is clogged and drains reeeal slow, I'm having someone come out tomorrow to clean the drains, any other thoughts or opinions? Thank you!!