As recommended, you should change fluid/filter. However, you've probably properly diagnosed the issue to a worn intermediate servo. They aren't that hard to replace, but I've only done it with the trans lowered/disconnected from the car.
The servo may be servicable while still in the car; it's a little tight, but it maybe can be done. You can get a billet servo from TCI.
I have the servos I pulled from my trans when I upgraded to TCI servos. Both of them have roughly 25k miles IIRC; would sell those for a reasonable low price, or just one.
Also, and maybe something you'd be looking more for, I'm pulling and selling my beefed-up 5r55s (going with a 4r70 for racing purposes). I bought the trans used, have put 8k miles on, with billet TCI servos and hardenned input shaft, and flexible dipstick tube in the pan. The trans internally has performance/build modifications, but I can't be sure how much power it can really hold. I'm putting 500 to the wheels and have had no issues with it whatsover. I'm adding a 'moderate' amount of power to mine and don't want to take the chance of trashing a perfectly good upgraded trans.
I'll be asking $900 for it when it goes for sale (it's coming out this coming weekend). Shipping these is a bitch though, and can run over $300 just for shipping.
Regardless, get it fixed before it gets worse!!!