MIDI clock does not synchronize precisely

Hi, I am new to Ardour 8 with Windows operating system and have the following problem:
I want to control a Behringer RD8 drum machine with midi via Ardour. Ardour should be the master and the RD8 the slave. I have an ESI M4U Xl MIDI interface for this.
Recording takes place via a Focusrite 2i2 interface using a jack.
I have checked “Enable Mclk generator” and connected “MIDI connections”: Ardour Misc - MIDI clock out with Hardware - MIDI Out Chanel 1 of the ESI M4U. (And of course connected with MIDI cable).
The problem now lies in this. The clock of Ardour and the clock of the RD8 diverge over time, i.e. they do not remain synchronized. It is very noticeable from 2 minutes onwards and then gets worse. I can change all BPM inputs in Ardour and the RD8 also accepts them, e.g. 144 and the RD8 plays 144 first. So there is a connection, but it is not precise over time.
I have also tried it with the Jack audio driver and as a connection via USB: Same problem. To test the hardware RD8/MidiInterface/USBInterface/cable, I tried the MIDI synchronization and with an old version of Cubase. The RD8 ran 100% synchronized as a slave with the Cubase clock.
Unfortunately I don’t know what to do… can anyone here help me?
Vincont