trans sticks in 3rd
 

I have a w126 1988 300 SEL with automatic trans, I think 722.351. The other day I took a trip into the mountains and downshifted into 3rd on a couple of steep hills. Coming home, well past the hills and after a stop at the gas station, I start to pull away from a light and I notice that my engine is revving and my RPM's are at 4000 at 35mph. I tried downshifting into 3rd and back into drive; no luck. Stopped, checked fluid (changed fluid and filter 20,000 miles ago) and the fluid looks good. Got in, it shifted fine, and did so the entire 100 mile trip home.

Any thoughts? Perusal of the forum indicates sludge in the valve body screens perhaps? Where would I get the gasket needed to replace the valve body? One post http://www.peachparts.com/shopforum/tech-help/201899-try-repair-when-your-xmission-wont-shift.html suggested that the valve body gasket was a single gasket and cost roughly 8 bucks, but when I look on fastlane it is a kit consisting of like 12 gaskets and costing 80 bucks. http://catalog.peachparts.com/ShopByVehicle.epc?q=1988-Mb-300sel-Transmission&yearid=1988%40%401988&makeid=MB%40%40MB%40%40X&modelid=300-SEL-006%40%40300SEL&catid=J%40%40Transmission&subcatid=J5820@@Valve+Body+Gasket+Set&mode=PA
Am I looking at the right part. What exactly would I need gasket wise to just open the trans, change the filter, clean the valve body filters and put it all back together. I figure that would be a good place to start.

I also read about superior shift kits, but that sounds like it is more for flare between gears. Any other thoughts? I know my way around an engine, but have done very little to auto trans other than just changing the filter. Thanks in advance for your input.