I *think* the best thing you can do, concerning cleaning inside is, to leave it alone other than gentle puff of air to blow off lose dust. Touching mirror is a big no-no. It's a front surface mirror. You can easily scratch it. Excessive pressure, manual or air, against shutter, is also asking for trouble. It can come off track and you won't be able to fix it yourself. If it isn't really dirty, you may just as well leave it alone.

About cleaning, on DSLR, you do MIRROR UP and SHUTTER OPEN to clean the sensor. You have none of that in film DSLR.

Concerning your auto-focus, try cleaning the electrical contact on body and on lens. (those few pins on both). Also, double check battery is fresh and contacts are clean. (battery case and the mating contacts on camera body). Plastic erasers are great to clean these.

If you do the reset right, the LCD display will blink. If you didn't see that, then it didn't work. Try again. If you really didn't do anything and it works sometimes and it fails sometimes, then you have a very difficult problem to find and possibly repair. If it were my newly acquired body, I'd be thinking about returning it.... Even with lens cap on, autofocus will try to find focus. So if it did nothing in dark, it didn't work at all.