you're torque converter is getting worn. drive it till it dies then replace. my truck was doing the same thing. i had the tranny rebuilt, put a shift kit in it and new torque converter. $1600 (good deal) then drove it from california to michigan. with no problums. as a matter of fact, she will almost bark the tires gong into 2nd.
shift pattern: D.....2nd.....3rd.....4th.....5th.....6th
4th and 6th is torque converter lock up
everytime it hit 4th and 6th it would "chatter" RATATATAT!!
what yours is doing is when you give it some gas it will slip then...BAM! the clutch plates in the torque converter will catch. couse they are worndown.
do you want to know the reason why? on the freeway, everytime you let off the gas the dam torque converter will disengage. to "HELP" slow the truck down...well,
.....SMOKE!! THE CLUTCH PLATES!. ford and there good idea's
now the reason i had my tranny rebuilt along with a shift kit and new torque converter is couse the tranny rebuild kit comes with a delite kit. that delite kit keeps the torque converter from engaging and disengaging everytime you let off the peddle.
when i got my truck back from the mechanic, i got on the freeway, got up to 65 and let my foot off the gas. the truck didn't downshift like it did before. this tranny and torque converter should last a long time.