I have to route out a 3/8" deep grove apx 4" by 18 "

problem I am having is after the first few inches the router is now only
resting on 1/2 the platform and it rocks forward and does n asty things--i
hve larned that you have tto set up guides with this thingh

Two much safer options:
1) If this is NOT a stopped groove, meaning it goes from end of board to end
of board, I would use a stacked dado cutter on the table saw.
2) Perform the rout in a router table.

is ther some sort of attachment that would fit on the base of the router
that would act like a footing and is adjustible so i dont have to keep
foolin with placing guides?

You could make your own base that is large enough and screw it to the
router.