At a previous brewing job, I experienced a similar problem with newly-delivered kegs. I found it best to allow shaken or agitated kegs to sit in the cooler for 24-48 hours before dispensing. This is very similar to opening a shaken can or bottle of carbonated beverage.
I would not recommend adjusting the pressure on the beer to fix a draft problem. Pressure should be set based on the temperature of the beer in storage and the desired volmes of carbonation. Other variables, such as line restriction, should be adjusted based on the keg pressure, not the other way around. If the account increased the pressure on the keg, decreasing pressure at the regulator may not necessarily drop the keg pressure, especially if the CO2 check valve in the tavern head is working. Make sure to vent the keg headspace (most tavern heads have a vent ring) when dropping the regulator pressure.
Verify the pressure guage on the regulator is working. It is possible the guage is broken and displaying the wrong pressure.
If the keg has sat for an extended period of time at higher or lower pressure, the account has most likely changed the amount of carbonation in the beer and the keg should be replaced.
Check to make sure the dispensing temperature is at or below the storage temperature. It is possible the glycol pack for the lines is malfunctioning or is low on glycol. If the temperature of the beer increases during its path from keg to faucet, CO2 will most likely separate in the lines, causing spitting and foaming at the faucet.
Another potential problem is a defective spear or coupler. I recommend looking at the beer line on the top of the keg for CO2 bubbles while the beer is being dispensed. If bubbles are forming in this line, they will also be forming in the draft line. You may wish to immediately replace the keg and tavern head--not only as a sign of good faith to the customer, but also as a way of identifying the source of the problem.
Beer stone or deposits in the line can create nucleation sites and cause CO2 to come out of solution. Thoroughly clean the line with caustic and acid (with a fresh water rinse between and after). Other sources of turbulence in the line, such as kinks, can cause CO2 to form in the lines as well.
If all else fails, try swapping the keg with another brand (I recommend using a new keg and cleaning both lines when doing this). This should help determine if it is the keg or the line. Remember to swap the tap handles!
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