cbo Box multiple calls not possible

Dear friends I need your help,
cboBox ist connected to an access - database, having two fields:
Current-No and remarks. the cboBox and the textfield are both on the same
form.

By a Mouseclick on the box the remark related to the current number in the
database shall be displayed in the text box.

This functions fine 3 times i.e. for three clicks mad at random on the
cbobox. On a forth click the system breaks down.
When  the form is unloaded and set = nothing and called from another form
after every click there is no problem every click shows the related text.

I tried this example with and without refreshing the form as well as the
controls. What is the reason for this behaviour of the control elements and
how can the same be avoided?

Thanks in advance
regards,
Knut