1999 Subaru Impreza Transmission Kicks hard into gear

My 99 Impreza Outback Sport has some issues with the transmission. When putting the car into gear at a stop, it hesitates for some time, then kicks very hard into gear, usually accompanied with a loud clunk. It shifts quicker if given gas, but the jolt and clunk is even worse if you do that.

If I have the brakes on, it will be less severe. Often times, the car will even roll backwards and refuse to go into gear until given a substantial amount of gas. I've taken to holding down the brake and gas when this happens until the car shifts into gear.

It only does this when put into gear from reverse or park. When the car is driving, the transmission works fine and does not have a noticeable slip, and it does not do this when taking off from lights or signs. Also, when the car is warm, the problem is far less severe.

The problem seems to have gotten worse recently. I've already tried changing the transmission fluid, diff fluid and the trans filter, along with various bottled additives, and at best I have gotten a temporary reduction in the problem.

I've heard various responses from this question, ranging from "trade it in, you're screwed" to people suggesting it could possibly be an issue with the solenoids/shift linkage that could be inexpensively fixed.