HI, Ive got a 1986 300sdl that I have fixed some vaccum lines on and changed transmission fluid in. Transmission shifts perfectly in all gears when cold but after driving awhile it gets stuck in either 2nd or 3rd gear. Most of the time shifting it back into N and back to drive will get it to upshift back to 4th. Would like to know what to look at and how to test. I have 5in vaccum at transmission at an idle. Any help Please and thanks.