From memory TCS = Traction Control System
(A-LSD) SLIP = Active - Limited Slip Differential (your rear diff)
Don't think it will have anything to do with the spark plugs, get RE Customs to check out the sensors / system, run a diagnostics test to isolate the errors and find out why the engine light comes on and off.
Spark plug miss fire is usually something to do with the plugs itself / spark plug leads, then get the distributor / alternator checked out.
You car is getting close to a 100,000 km service... Yes it is a big one.
Timing belt <--- Usually owners in Japan change it around the 60,000 to 80,000 kms mark, but just to be safe, change it so you are 100% sure it is brand new / Won't need to be changed for another 100,000 kms.
Fan belt <--- Usually changed when the Timing belt is done.
Spark plugs
Oil
Oil filter
Air filter
Fuel filter
Gear box oil
Radiator coolant flush
Check entire engine
Check entire drivetrain = brakes / diff / suspension / tyres / clutch / gearbox etc.
+ Whatever is also due. LOL!
There is a thread on a 100,000 kms service, some of the R34 GTT owners can dig it up for you.
Like what R34mac said.
OR Check the hose clamp and re tighten it... It may just be a little loose... Or when you take it down to RE Customs, get Ray to check it out.
Keep in mind, on a hot day, turbo cars will not be as responsive with boost and some times you may not even get full boost, due the the heat.
Since you only have a side mount intercooler, the R34 GTT ones are pretty good for what they are, but when 35+ degree C days come, you will notice a slight lag.
Again normal due to hot outside temps.