Your problem sounds more like an idle surge than a transmission problem to me...this will occur with the transmission in "Park" as well, but it's not as pronounced as when the engine is under load (i.e. with transmission engaged).
Your symptoms are while stopped with the transmission engaged, the idle speed oscillates, and you will note the corresponding actions with the tach and oil pressure gauge needles bouncing in unison (I don't have an economy gauge).
This is especially pronounced if you place the tranny in "R"...feels like you are playing with low-rider hydraulic suspension switches
The surge is caused by a number of factors, with the primary ones being a vacuum leak, a bad air mass sensor unit, or a bad coolant temperature sensor.
I have been tolerating my surge problem for about three years!
I have replaced the air mass sensor (expensive $$$) and the coolant temp sensor, and the problem still persists. I have also replaced some broken vacuum actuators and sealed off suspected breaches, but so far, to no avail.
Replacing fuses might cure some other electrical ills you may already be having...but it won't cure this one.
I would hunt down vacuum leaks first. It's the cheapest route...