Sounds to me like your head gasket. Last year before I bought my VX, the previous owner spent almost $3,000 getting the heads fixed. The mechanic that worked on it apparently didn't machine the heads down properly nor did he tighten the bolts as much as he should've.....my baby has just now gotten out of the shop getting the heads fixed again. Luckily, I paid for parts and that's it. My mechanic is one of my best friends. I didn't know my head gasket was blown, there were no symptoms, except for a little overheating, under the same conditions you've experienced. There is something called a "block tester"...costs about $45 from NAPA. Follow the instructions and you can find out if there is combustion leaking into your antifreeze. THAT will happen before antifreeze leaks into your oil. Once antifreeze leaks into your oil, your on your way to bigger and more expensive repairs. So catch it early! You can buy this block tester and check it yourself or let a mechanic charge you $300 to do the same thing. At least if you do it yourself, you'll know what your mechanic needs to fix instead of them charging you to look at it, test it, tell you what's wrong, and then finally charge you for working on it. They might "find" a lot wrong with it, charge you for it, and you'll still have the same problems because they didn't fix the correct thing in the first place.