1995 Ford F-150, 4.9L 6 cyls with Fuel injection,

Failed emmision test with high hydrocarbon (two times higher than the
allowed limit).

The truck had its ICM module replaced about two years ago. The ICM
failure was gradualy starting with rpm dip to almost zero while at
idle, and this became more frequent before the complete failure of the
module.  An after-market module was installed (not made by Ford) and it
was working fine for a few months. Then similar things happened again,
i.e., the rpm dip to very low while at idle. This may happen once a
week or so, not very often.

The truck was not on daily use for three months before the emmision
test (probably driving once a week for 20 miles).

It is suspected that the ICM replacement might be the cause of the
emmision failure.

Any idea and suggestions are highly appreciated.