lost power while driving

it happens to lose power when I switch on the car air conditioner (the car engine stopped at all the moment I press the brake pedal). same is when I am going uphill, I cant accelerate even if I press the gas pedal at maximum.
I have a Golf, '96.
I would like to know where to start diagnosing the problem.
thank you

I'd suggest you start by confirming there is no vacuum leak, then perform a tune up (including fuel and air filters). If that doesn't fix the issue, check fuel pressure and make sure the idle air control system is functioning properly.

-Rod

There is a relay and/or a vacuum-actuated valve that is supposed to increase the idle speed of the engine when you turn the A/C on. If they are not working, then the A/C load may be too much when you are at a stop. Check the relay, vacuum lines, and wiring to/from these parts.