There is a little bit of a hump when cold due to raised idle speed, but if more than hardly noticeable, the cause will no doubt be the turbine speed sensor, this is located on the end of the gearbox visible facing the nearside wheel, got a 2 pin plug onto it, these always pack up.
My advice it is NOT diy!!, easy enough to do just take out and swap over for new one, fine in theory, reality says it is stuck solid, and the outer will break off leaving the guts, needing drilling, but drill too much and major expense follows, so if not used to these, just stump up 1 hour labour cost at auto box repairers, they will also be only too happy to test drive the car, in case it is something else.
If using for towing or thrashing get a air/trans oil cooler, seriously needed on these, will help box live a lot longer, as will regular fluid changes.
The other fault that early ones could suffer from was the clutches, specifically the clutch basket could snap in places, upgrades should all have been done under warranty but a lot missed this, and the gear selector switch(inside box) goes potty, but you would be having sudden downchanges to 2nd gear at 50 mph etc, totally undriveable like this.
tabetha