I would check your exhaust back pressure.
the brake booster works on engine vacuum (unless you have hydroboost) so if you engine is running poorly and you open the throttle to accelerate you loose your vacuum. So, it must be pretty bad if it is affecting your brake pedal.
turning on electrical loads ( the lights, the heater...) make the engine have to work harder because they make the alternator work more putting a load on the alternator belt slowing the crank pulley.
you might also check your cylinder compression...
you should also look at your injector spray pattern. you should have throttle body injection so with your air cleaner and the air cleaner riser removed you can watch your injectors spray fuel down the throttle body.https://www.youtube.com/watch?v=5A4a5OOGqPo