generally slipping can be the result of a mis-adjusted inching pedal but since you say it is only in forward i tend to think it could be the result of worn fwd clutch pack or some problem in the fwd circuit in the trans.
First thing i would do is get a pressure guage and hook up to the transmission control valve and take some pressure readings. This will tell you if it's pressure related or mechanical.
In most cases where i've seen a forward slip it has usually been the result of the operator pushing loads causing the clutch pack to wear out prematurly.
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