Timing problem with a VX

[GasSavers_BIBI]

Well I might just buy a VX with less then 100k miles on it. The car is clean, no rust, and this VX seems in real good shape. I'm going to see the car tonight, 2 hours trip to go. The only problem would be the timing the owner told me. She told me that the car need a TIMING. She described it to me like this: when crusing in gear, the car rpm will suddenly drop from 2500 to 1700 lets say, and then go back to normal. But the car never stall or anything.

I'm wondering what exactly will I have to change or do and what price will it cost. Is that a valve adjustment, the O2 sensor, ect. I aint gonna buy the car until I find out about that. Anyway I'm going to try it tonight and will have more info to give you to try to help me.