Vibrations, help

Took my 98' auto for an ATF change last week to my trusted garage which is about 40 minutes away. On the way there it developed a strange vibration when travelling between 30 and 40mph. This was the first time i'd driven it more than a few miles. Got the change done, and drove it to get valeted. It didn't produce the same fault on the way there, only a 10 minute drive. It stayed there over the weekend, and I picked it up today. Decided to go the scenic route home and give it a bit of a blast and lone behold 20 mins down the road when I had to slow down for a village, the 30-40 vibration came back. Engine revs don't fluctuate like the feeling would make me think they would, and can feel it through the steering wheel and the seat of my pants. I don't notice it if i'm accelerating through those speeds, just if i'm sitting at them under low load. Car had sat for 2 years before i bought it last month. TIA.

Was it all ok before ? When they did the ATF change did they do it like it says in one of the threads and what did they use ? If they used the incorrect fluid it will cause a problem.
Sure someone else will come along and give you a more information on this but you could try searching and have a good read.
Hopefully you will sort it out :)

It did it on the way to get changed, that was the first time I'd really drove it. Fluid was the correct type, bought it off here. Tried to search for same fault, but can't seem to find anything that matches symptoms exactly

Light throttle, 30 to 40 mph and 4th or 5th is indicative of worn or glazed clutch packs. If you have just had an oil flush with Amsoil then the best thing you can do is drive it around. Amsoil has cleaning additives in which can help. If you only had a sump drain and refill then I would suggest you try a full flush. If someone before you used Dexron 3 and not SPIII then that can cause glazing to the clutch packs.