Hello again,
IMHO until you disable the massive Log-In and Log-out, your ms sql server will be working in this way ...
Maybe you should increase the size of your T-Log.
The sql server in simple recover mode, periodically truncates the t-log (this doesn't shrink it or reduces the t-log file size).
Anyway, maybe this log truncation is delayed for some reason.
Usually when the t-log becomes full, in the windows event log appears a message telling to check the log_reuse_wait_desc column of the sys.databases view, in order to know the reason because the t-log space can't be reused (a t-log truncation can't be done)
Best regards,
Pablo