These Cars the x300 Xj6 my car and i hear xj8s have the same problem just go into a mode where the car cant make it up hills or rev ubove 1700 when in the drive dear selection. My conclusion is the cars are not designed for hot heat, PROBLEM coil packs are individual and inside a hot hot motor in separate hot chambers, they fail when the car gets into traffic and no wind. Now that its winter time it runs great no misfires no check engine light. In FL on a trip stalled out on the highway, could not get it to go very fast because of the coil packs heating up. and its a problem that is hard to figure out took me months. The car is not designed for climates higher then 80F. if you live in FL take the useless flaps off the radiator for more air flow all of them, they are made so in the winter time the car wont fully heat up to normal so much so the heater wont even work well. so rubber flaps by the company were installed to limit the air. Take the top cover off the motor to expose the coil packs, try to find new coil packs and replace them if you can afford it, but if you take the panel off the top and the to get m ore air into the radiator it will significantly lower the rate of the problem, My car now that its the winter time and 30F outside runs with no check engine light no missfires no problems. Its just not made for hot weather. And the dealer ships know of this problem, but they will not help you and they will pretend it does not exist.