Car stops when I slow down

I have the car listed above and before I purchased it (about 2 months ago) it was sitting for a while and the battery needed a charge and there was a issue with the brake light. The rear brake lights (the ones that come on outside the vehicle when pressing the brake) were always on and that was fixed by placing a bushing above the brake pedal. While testing out the car I saw the check engine light would always be on and sometimes while I was driving another light would come on briefly then turn off. About a few weeks later when I went turned the car on (which I was still driving almost daily) it would not turn on, it would crank but the battery light would be on, to solve that issue, I checked the emergency brake, and also I put the car in reverse, then back in park while pressing the brake then turned it on and it ran normally. Then about 4 weeks ago, the car wouldn't turn on after previously driving it hours before. We at first thought it was the battery but later inspection showed that either there was a terminal issue or something else was going on. So we took the battery to advance auto and the battery was charged and perfectly fine and looked brand new. So we replaced the battery terminal that seemed loose and also the battery to be on the safe side The car still would not start after replacing the battery(the battery later ended up being a issue due to the terminals not being tightened) and it was believed to be the starter (due to the hammer test) but the starter was replaced and the car did however turn on and drive but the battery and brake light would come on and the car would "stop"(engine would turn off yet lights were still on and car is in drive) when I would slow down or press on the brake completely. The car would drive yet I would have to firmly press the pedal to keep it from "stopping". The car would also make this clunking noise also when I was driving, it was at first believed to be the alternator but in turn it was actually the vacuum/tubing that connects to the mass air flow sensor had a big hole in it (which was never a known issue prior) so that was replaced by a mechanic and the car ran perfectly since. However a few weeks later there was a issue with the battery terminal not being tightened properly and over that time slowly became loose enough to no longer provide power. Which was also fixed. I continued driving the vehicle about a week ago I decided to get my car diagnosed and possibly an oil change as well. And the car was in horrible shape I was told, the harmonic balancer was gone, the belt tensioner and both belts needed to be replaced and the car needed the valve cover gaskets replaced, a tune up as well as a engine decarb was necessary all which I have yet to fix at the moment. The next issue I had with my car was while I was driving and slowed down at a stop light the car stopped yet again but this time it would not turn on when I started it. The lights were on but it would not turn over. I was wondering was the balancer and belts the possible issue or could it be something else?