there is a known problem with third gens regarding this. when drivnig in overdrive, at 45-60mph, and the cooling fans come on, there can be a condition that the vibration of those fans sound like the vibration of the engine detonating to the knock sensor, and it retards timing, this changes engine speed, and poweroutput, making the converter unlock, then the conditions return to normal, sensors tell the converter to lock up, then the knock sensor retards the timing and so on and so forth. if your rpms are dropping by a few hundred, then going back up, then dropping a few hundred, then going back up, that is more likely than not the problem. however, the shitty news, there is no fix for it.
the only thing i have heard to try is buy a new fan motor, and the vibrations may be different enough to not cause the problem. i know mine started diong it, after i put a new fan in, after the old one burned up. you could also try insulating the fan somehow, but im not sure how you would do that. its already monuted on rubber stoppers i think. also, when the car is doing this, watch to see how many rpms drop off. then downshift into third. if the downshift is more rpms than the drop you saw before, than it is the converter, if the rpms are the same then its actually an internal trans problem with the fourth gear servo or something, i forget what its called. cuz its actually dropping in and out of fourth gear. could also be a bad TCC switch. the only reason i know all this, is im having the same problem with my RS, and have yet to track down the problem. ill let ya know if i figure it out.