Canada 411: Something is fishy.
You said all tests gave good results (mixture ok, O2, plugs are OK, ignition is perfect, valvesOK, and everything else.
???????????????????????????????? and you get 13mpg.
Have you had these tests done at idle only? or at different RPM.
With and without load on the engine?
You said the engine is missing at a certain RPM and you smell gas: Could it be your fuel/air mixture goes wild under certain conditions (vacuum leaks, leaky injectors, fuel distributor?) Have you eliminated all possibilities of fuel leaks(filter, pump, fuel line, which wpould happen only when the car is moving or with the engine running?
How's you air cleaner? Any restrictions in the air intake at speed?
You have to take this problem with a different approach instead of throwing parts at the problem.
Do you deal with a competent mechanic?
JackD
Montreal