Have a kegerator home built system for 1/2 barrel beer in the garage. Recently had a keg of beer that was foamy no matter what we tried to do. Served it low temp, 37 degrews, and almost no CO2 pressure. Tried serving warmer, with more pressure, still tons of foam.
There are a few things you can check with you keg. You said it's a home built system, yes? So is this a mini-fridge conversion, chest freezer/keezer conversion, or some other kit?
Anyway, what length are the beer lines? Generally you need to balance CO2 pressure against beer lines to avoid consistently foamy pours. If your system isn't properly balanced the beer will always be foamy, on every pour. I have 5-6 foot lines in my kegerator and it works pretty well for me, when I was having foamy pour problems everyone suggested replacing them with 10 foot lines just to make damn sure the pour wasn't too violent. Also making sure the lines are clean. Any beer stone or other sediment in the lines is going to cause foam.
Serving warmer with lower CO2 would also be a better step. One frustrating thing about these sorts of adjustments is that it takes time for equilibrium to be hit. So if you lower the CO2 pressure, the headspace is still pressurize. You'll need to vent the keg via the pressure release valve. Same deal with raising the temperature. Once the beer actually comes up to the new temp, there will have been CO2 driven out of solution that you need to vent from the headspace via the pressure release valve. It's a pain. Regulators will also often have their own pressure valve, which you should poke and vent slightly when setting the pressure...especially when coming down since excess pressure in the system is going to mess with the reading. So vent, watch it rise to the actual pressure, then adjust, vent again, and watch. It takes some time but generally once you set it there's very little change required.
It's hard to tell without seeing the speed of your pour.
Definitely check the keg temperature itself. As a homebrewer I reuse kegs, so I have stickers on all my kegs that have a little temperature line. These liquid crystal thermometers, like this one, are pretty cheap.
You can also check the ambient temperature using a regular thermometer and a small glass of water. You should check temperature at multiple spots inside the kegerator, so you could fill some tumblers or shot glasses with water and place them at the bottom of the kegerator, then maybe on top of the keg, and then maybe a back shelf if you have anything like that. This lets you know if you have any hot spots which you can reduce by reorganizing tubes or tossing a small computer case fan in there.
Also, if you haven't disabled the dome light in the fridge, do so or find some way to make sure the switch stays down beyond a doubt. When I first built my kegerator I was getting incredibly foamy pours and a huge contributor wound up being the dome light kept coming on and heating up the tops of the kegs and the beer in my lines.
Another final check is the temperature inside of the tower. If you have no way for the cold air to get from the body of the fridge into the tower the lines in the tower will get warm and cause foamy beer, though if the CO2 pressure is set correctly, this would likely only result in the first 1-2 pours being foamy, then it calms down until the lines warm back up. A lot of people will put fans or run copper tubes up the towers. I filled mine with insulation.
You were right to lower the temperature. Beer (or any liquid for that matter) is better at retaining gasses at lower temperatures. Therefore: cold beer = less gas released = less foam produced.
Occam's Razor says your keg is probably warm. I'd check if your temperature is correct. Make sure the temperature is being checked from the bottom of the keg, as this is where your beer is coming from.
I'm assuming you're pouring properly, (this includes using a clean, unfrozen glass with a smooth interior).
Aside from that, I'd make sure all the parts of your system are clean and in good working order.
Lots of good, in-depth answers here. It looks like nobody (edit: looks like Soloem did, actually) mentioned the obvious: close your supply line and bleed excess CO2 from the keg.
Sometimes a keg will come to you highly carbonated and you just have to bleed it. Sometimes you have to bleed it a few times (before you begin dispensing for the day).
As long as you don't have some physical problem with your setup (long lines, warm temps, leaks or something crazy) you're just facing an over-pressurized keg.