As my CRX Si (89, standard shift, no engine mods, 115K, not a daily driver) comes to normal driving temperature the engine revs, on its own, to around 3500 RPM. (This happens both when I'm sitting in neutral at a light and when I'm moving.) The revs happen three or four times, then the car drives normally.

My usually decent mechanic has looked at it twice and gotten nowhere.

Help, please--thanks much.