We have a stand-alone, wall-mounted A/C unit that exhausts to the outside and is hard-wired to a wall switch to turn the unit off and on. The unit is about 10-12 feet above the floor and the switch is about 6 feet above. Lately I have noticed that the room has gotten pretty warm, indicating that the A/C unit isn't turning on. If I turn the wall switch off and then back on again, the unit turns on and runs normally until the next time this happens (sometimes it happens on the next cycle and sometimes it doesn't happen again for a day or two - there's no obvious pattern.).

The question is, does this sound like an electrical problem with the switch (loose wire, for example) or with the unit itself? Should I call an electrician or an HVAC guy. Any troubleshooting/diagnostic tips will be greatly appreciated!

Try giving the unit a good slap next time. If it takes off, it is a good bet the trouble is inside, most likely the temperature control. They aren't that big of a deal to change, if you can find the right replacement. Shut the power off. Remove it and take it to an appliance part supply place, or maybe where you bought the air conditioner. Take the model and serial number. If it is Sears, you must have that to even buy a screw to fit it.

If that doesn't do it, try tapping, but not shutting off the switch. It that seems to do anything, go buy a new switch, shut the breaker off, and replace it. A new switch is only about a buck and a 5 minute job to replace. Even if the problem is a loose wire at the switch, that will be a cheap fix.

Note, hitting things may be an acceptable way of finding problems, but should never be seen as a fix. Bad connections are dangerous and need to be fixed.