If the timing is 15 BTC, then the timing belt is installed properly.
Does the check engine light turn on for two seconds when you first turn the key to the II position? Does the CEL turn on or the D4 (or S) light start flashing when driving? Check for error codes anyway. See how to get CEL codes in the DIY section. Post any codes you find.
I would clean the throttle body and both sides of the throttle plate with throttle body cleaner. Is there some slack on the throttle cable?
You may also want to read this thread about fluctuating idle. 1993 F22A Idle problem
Maybe reset the ECU after checking for error code(s) by pulling the 7.5 amp backup/radio fuse in the engine bay fuse box for a minute.