I drive a 1993 Toyota Corolla with 212,000 miles on it. Sometimes it'll do this thing where after I shift it into a gear, it'll accelerate normally for a moment, and then suddenly my RPM will shoot up like I'm in neutral. If I let off the gas and then accelerate again it goes back to normal. It mostly does it in 3rd gear, sometimes in 2nd, and it's done it once or twice in 4th.
My guess is that the gears are slipping because they're worn, but there's a few things about that that confuse me. For one thing, it doesn't do it consistently -- it'll do it constantly for a day or two, and then it'll go back to normal and not have any problems. The other thing is that I would think if it was a gear problem depressing the clutch would fix it, and the accelerator wouldn't have an effect, but letting off the gas momentarily will fix it.
It's probably something unfixable, but I'm just curious as to what exactly it is, because it freaks me the hell out whenever it happens.