I had 2 tii's and if I remember correctly the linkages had to be set up first. That includes the valve on the pump that gives it extra fuel when cold (if that is malfunctioning you can be driving rich).
If you suspect it is the injectors, those are easily removable and you can either clean them yourself of have a shop do it (spray pattern test, cheap).
The fact that you are getting 90mpg per tank tells me you are running stinky rich.
I'm not a mechanic, but there are only 3 things involved here..
Fuel
Spark
Air
How clean is your throttle plate? is it openning 100% at full throttle? another easy thing to check.
Have you run it w/o the air intake ? Maybe something is clogging one of the 2 filters...
Good luck,
Robert
edit.. one other thing.. if the timing is too retarded you will experience those top end problems, and overheating as well.
last note... Ignition wires... check the resistance and make sure they are all within spec. (replace cap and rotor to get those out of the equation)