If you've got any diagnostic issues your warning light (yellow engine) would be on all the time, on the right side cluster. If it is on you can check for the error code here
http://www.mkivsupra.net/vbb/showthread.php?t=35920
I suggest you disconnect the battery for 10/20 mins. This resets the ECU and gets rid of any old stored codes. It may also alleviate your symptoms......................
You would be better off cleaning your 02 sensors, disconnect the battery as above, then see what happens.Also, change your spark plugs.
DONT waste your money on a "diagnostic" HTH.