Peugeot 207 Auto gearbox faulty?

Hi there
My peugeot 207 automatic gearbox is faulty,

1st drive of the day like clockwork I'm Driving along at 50mph and then the car clunk's and seems to drop a gear and the car beeps and on the stereo screen it shows " gearbox faulty" and next to the "D " a snowflake symbol appears with the s mode symbol too.. but it then drives really sluggish.. however if you stop and turn the car off and back on again it runs like there is nothing wrong!!

I have been in and out the garage 4 times..frustrating!! They replaced the multi function switch..but didn't work..and they had it on the diagnostic machine 2 more times after fitting the switch and same fault kept appearing..took it in again this week and all they did was adjust this so called switch they had fitted 2 weeks ago..and low and behold its still not fixed..I'm honestly at my witts end!!
I notice a lot of people seem to be having success with the oil pressure solenoid valve.. would you advise that this would perhaps cure mine?