My organ students often have a trouble keeping a steady tempo. They either speed up or slow down depending on the situation.
Keeping a steady tempo has a lot to do with keeping a steady pulse.
How do you do it?
It's simple: Count the beats in a measure out loud.
That's where the students start to argue:
"Can I just count in my mind?"
"Yes, you can but you won't be able to tell whether your counting is even."
"But it's hard to say the numbers so you could hear them and play at the same time."
Yes, it is but do it until your sense of pulse improves and you won't regret it."
There is something magical about being able to listen to your own voice.
It's like observing another you which turns you into your own guinea pig.