I had that problem with my SL, but when it goes into limp mode you really know it. The engine is running on 3 cylinders. There are many factors that will cause this. All the ones previously mentioned are correct, but the most common reason for the I6 to go into limp is due to a misfire. Turning the engine off and restarting will resolve temporarily.
First thing to do is figure if it is related to load, 30mph is a certain RPM, with the car in park, run the engine to that RPM, does it start to sputter and misfire? if so it is not related to load or drivetrain. Check the plugs, coils, and ignition wires. If this doesn't resolve check your throttle actuator, and then your speed sensors. Have you replaced the wiring harness? The 95 has the harness issue, be sure to replace it before it ruins anything else. Also check your vacuum lines. Was anything done to the car prior to this condition? Oil change, repair? These are places to start.
Good luck!
Brian

__________________
Brian W. Heitman

www.MBCA.org/northtexas

Join MBCA!!!