almost certainly
Therefore, the monitor was almost certainly the source of the problem.
had a stronghigh propensity to be
Therefore, there is a high propensity the monitor is the source of the problem.
was in all likelihood
Therefore, the monitor is in all likelihood the source of the problem.
most probably was
Therefore, the monitor was most probably the source of the problem.
in all probability was
Therefore, the monitor was in all probability the source of the problem.
there was every chance that
Therefore, there was every chance the monitor was the source of the problem.
snowball's chance in hell
Therefore, there was only a snowball's chance in hell the monitor was not the source of the problem.