1994 Buick Park Avenue Eninge redlines by itself

Hello, I have been working on cars for about 5 years and I am stumped on this problem. My girl friends car will be driving along fine except for a little shutter in the transmission which its had for a long time. Recently when driving, periodicly, the engine will rev up to red line and the car will gain a lot of speed on its on, cruise control is off. The only way to keep the engine from redlining is to apply a lot of pressure on the brakes. Even when driving and it does this, a lot of brake pressure wont stop the car because it wants to keep going. I have checked and lubed all the linkage for the throttle body and did not find any binding. Once this happens, once I shut the car off and come back in a little bit the problem is gone for a week or so till it decides to start accelerating on its own again, Any ideas on what could cause this as the car is unsafe to drive int he winter with this going on. Best regards, Jeff