1998 Volkswagen Beetle In 5th gear the engine shakes

Hi, what would be a reason for this behavior:

When driving my car and shifting through the first 4 gears everything seems normal, but when going to 5th the engine shakes, like it's coughing. It's worse if I go at low speed, at higher speed it ´s a little better but it's still very noticeable.

I was thinking maybe a spark plug is the problem, but it's weird that only on fifth it happens, while on gears 1 to 4 it seems to run normal.

thanks for any input!