flat spot in 3rd gear/entering highway
 

I have a 98 F-150 with a 4.6 K/n fipk dual exh. and a b/m electronic shift imp. Well,the
problem I'm having is not major but is a little disappointing. When I am on a freeway
entrance ramp,the engine performance seems to
drop off a little. This happens about the middle of 3rd gear and goes away after a couple of seconds,it then begins pulling strongly again. Any suggestions?

If this happens about 45 mph that is your torque converter locking up. The mph will vary depending on the amount of throttle your giving the engine.

that sounds right,but I've only noticed it
in the last hundred miles or so. before that
I do think it pulled all the way til I let off the pedal. almost feels like the timing
is off a couple of degrees for a brief moment
it's hard to describe really.

watch your tach. My truck seems to have a dull area from about 2200 to 2600 rpm. Then about 2700 rpm to 3900 rpm it climbs like crazy. This is called torque band and may be what you are experienceing.

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97 F-150 4.6 auto, 4x2, 3.55 LS, WMS Velocity tube, K&N, Dynomax Super Turbo exhaust with Echo tube.
Oh yeah, it's Oxford White with crome wheels, Legacy shell & custom grill!