i can sonic clean injectors in work, but its a right pita as you have to pulse them while soaking in petrol within the sonic bath.
prob best to go for a full service and a vagcom hook up - then go from there you never know vagcom may pull up a fault on the engine
oil,plugs, leads, fuel and air filter - clean the tb and run some injector cleaner in your fuel for a bit
how do you know you are down on power?