Grand Marquis Transmission problem

I posted this in the grand marquis forum but I didnt get any replies. The car is a 91 gm wagon with auto and 138k. The tranny fluid was changed by the previous owner around 4k ago.

The car shifts rough. The problem is really bad at 55mph. It accelerates fine up to that point but then it starts slowing down. If i give it a little more gas the whole car shudders. That stops only when I take my foot off the gas and when i start to accelerate again it will slam into the next gear and take off like a bat out of hell. This is repeated every time I slow down to 55 or less.

My mechanic cant find the problem.

it could be torque converter shudder...<tranny related>

but i think it may be detonation. <spark related>

fyi,

my 88 5.0l would do that until i did a full tune up.

Looks like the last owner had the plugs & wires changed about 8 months ago. Should I do it again just to be safe?

i would cap roter plugs wires and fyi only use moter craft plugs or autolight