In an automatic transmission this usually means the trany is low on fluid or the trany has internal problems.
Your transmission will slip and clunk or bang when changing gears, can have trouble changing thru the rand of gears, ans will eventually over heat and burn out.
Broken motor mount
The year and engine info would help.
Check fluid level Change fluid and filter Bring to a transmission shop
There are many reasons. Take it to a trans shop.
Inhibitor switch reduces engine power to avoid shock while automatic transmission changing gears
Automatic trans I take it, sounds like you may need to rebuild or replace soon. The gears could be warn and are starting to slip
The most common thing on convertibles (the vehicle) is that most of them have automatic transmission when driving. So instead of manually changing gears, it's automatic.
A bad universal joint is one possibility.
You need to use the clutch and the shifter smoothly together!
Are you sure it is the transmission that is clunking? It could be the u-joints on the drive shaft that are making the noise. When the u-joints on the drive shaft are bad, there is an audble clunk when you accelerate and when slowing down (basically the same motions you go through when shifting gears).