check vacuum and other stuff like these guys mentioned. If not those, then rev the car up to close to redline. If it shifts and there were no other issues present then it sounds like a valve body. My '87 had this issue ... chased vacuum issues for some time!

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Current cars:
1987 Mercedes 300D
1997 Turbo Nissan Maxima
2001 Lexus IS300 2JZ-GTE & 6 speed swap
2002 BMW E46 M3 - done ... needs engine management
2005 E55 AMG - mods started starts with a pulley and goes on from there!!!