Mechcanical help please

OK - my car has some kind of issue - wanting some feedback before I take it to the mechanic so I at least sound like I might know what's wrong with it....

firstly - the car.

1990 subaru legacy - auto 1800 85,000kms

secondly - the problem

This happens randomly it appears - dosnt seem affected by how fast you are going - only consistentcy is the revs and perhaps temperature of the car- which is always about 3000 mark in 3rd or fourth of the auto transmission. Car starts to shudder and lose any acceleration/power lasts for a few minutes at max then goes away - might not happen again for a week or two. When idling in middle of this problem the car is louder than usual and the motor appears to be moving side to side in a rocking motion. Car had basic service about 1500kms ago..

any ideas anyone? chances are I will not be able to replicate problem for mechanic as it is so random when it happens but i do fear it is part of a bigger problem to be...

cheers

to be honest car problem diagnosis is the hardest thing to do according to a mate of mine thats a mechanic, I dont think its the auto, something might be broken engine wise or threatening to break, take it to a good subaru mechanic is my advice but someone else might know.