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What causes a 250 rpm surge after 45-50 mph?

I am wonder what can cause a 250 rpm jump after i hit 45-50 rpms

It sounds like the torque converter lockup engaging/disengaging. The torque converter lockup on my 95 Dodge Dakota would engage at 47-48 mph.

Ed B.

Is the toque converter problem be seen in higher speeds as well. It jumps in speed as high as i can legally go. It jumps up 250 then falls to normal after a few seconds and jumps again 250 after a few more.