You've hit the classic trap for new players!
The thing about an overcarbonated keg is that it doesn't result in highly carbonated beer. The classic symptom is massive head and flat beer... now where have I heard that before?
To understand what's going on here, you need to understand a few things about how to balance a kegging system; it's not as simple as it initially seems. The primary consideration when balancing your system is actually the pouring resistance, which is the amount of pressure required to force the beer through the hose and out the tap. In your case, you have one of those picnic faucets from Ross, right? That tap has a really fat hose (wide inner diameter or ID), and a big fat, short tap that creates a lot of turbulence when you open it. What all this means is that your pouring resistance is really low.
OK, if you have a low pouring resistance, it means that you need to use a low pouring pressure to balance it. Either that, or replace the hose with a smaller ID hose or a longer length of hose.
OK, so we want a low pouring pressure (I have one of those taps, used to use it before I got my faucet, and I reckon you need a pouring pressure no higher than 50kpa, if not less). Now, having a low pouring pressure brings up another issue, one that you have touched on. If you carb your beer at a high pressure, then turn the gas down, the partial pressure of CO2 will be higher inside the beer than in the headspace. Now, gases always tend towards equilibrium, so if you have a higher partial pressure dissolved in the beer than in the headspace, it will slowly evaporate out of the beer which means that
a) your beer will flatten over time
b) you get gas inside the hose to your tap
a) is probably a bit annoying, and can really only be fixed by rebalancing your system (longer hose or smaller ID hose).
b) is also a bit annoying as you can get a messy, heady beer from the first pour after you haven't used the keg for a while. Subsequent pours should be unaffected though.
Have I confused you yet?
If you're still reading, I'll tell you exactly how to fix the problem now.
1. Turn the pressure down! Try 50kpa.
2. Burp the keg every half our or so to release more gas. You can theoretically rock the keg to speed this up, but I've found you just end up spraying foam out of the valve.
3. Try pouring a beer. As you burp the keg over time, the carbonation level and head pressure of your beer drops, and your chances of pouring a good beer will go up.
Really, all this is about learning how to balance
your system. I think that seeing as you want nice sparkling beers, you're going to need quite a long beer line (with a small inner diameter), probably a good 3-4 metres. I have 2 metres myself, and I run my system around 50-75kpa.
I'll stop now
Oooh actually, if you want a better explanation:
Balancing A Draught System