I had the exact same problem. A rebuilt tranny sorted it out. I spoke to the builders and the said that they'd check out the oil and be able to tell if an oil change would fix it. The oil was bay dirty and there was a lot of build up on the friction plates. These guys seemed to know what they were on about so I agreed to a rebuilt tranny. I paid £1280. I wish I'd done it earlier as I had the problem all while I had the car and got it fixed 2 weeks ago. I sold the car today though. :-(