1998 V70XC RPM drop w/thump sound at 50-65mph

[undflyer]

Hi All,
I debated whether or not to post this because I want to be specific as possible.

Scenario: driving 50-65 mph on level freeway. Hit the gas to accelerate and the RPMs begin to increase ~300-500 rpm and then drop back down ~300-500 with a *thump* sound and then the car begins to accelerate. Only happens at this speed range. Any ideas?

thanks!

[abscate]

It sounds consistent with torque converter lock up, which is normal, in that range.

What is the history of the transmission fluid, or ATF ...when was it last changed ?

[undflyer]

I had the fluid drained and refilled about 8 months ago. Before that, I don't know (I have owned the car for just over two years). The fluid is mildy dirty and not the clear pink that it should be.