I have a 98 Subaru Outback with 142,000 miles on it. While cruise control is engaged, it has suddenly spiked rpms to between 6 & 7. This happens during straight aways without any obvious reason. I would understand if I was going up a hill and it was trying to maintain the speed. I have since stopped using the cruise control. My local Subaru mechanic has never heard of this phenomenon. Any suggestions are greatly appreciated!
What else happens when the RPMs spike? Does the car accelerate? Does it downshift? Is the engine actually reaching 6,000 - 7,000 rpm, or is the tachometer just jumping? Please supply more information.
The engine is actually reaching 7,000 rpm but the car is not accelerating. The engine races but doesn’t really accelerate.
If the engine is winding up to 7 grand with the car not accelerating your transmission is slipping.
And your mechanic is slipping too. Try another one.
Exactly right, Jay. If the engine speeds up in cruise control, it definitely means the engine has somehow become disconnected from the wheels, so the engine runs at maximum rpm trying to increase the speed of the car to the preset speed, and can’t.
Have you checked the automatic transmission fluid to see if it has any? 
When an automatic is very low of fluid, it will normally run most of the time, but at times can slip like this.
Otherwise, yeah, there is probably something wrong with the transmission. And, that doesn’t sound like a real good mechanic, since this is so obvious.
Your transmission or clutch is toast…