I'll kick off with what I'm using in Tapperesque.  If anyone has a better idea then tell us!

random_value:           dc.w 0

CODE IN VBI:


			rol.w	random_value    ; skew the random value counter


PROCEDURAL CODE:



;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;
;; Generate a random number in d0
;; (Range 0-127)
;;
;;

random:			lea 	random_value,a0	; vbl ROL counter
			move.l	$f00004,d0	; read video H/V offsets
			eor.w	d0,(a0)		; screw
			swap.w	d0		;  around
			add.w	d0,(a0)         ;   with
			add.w	d7,(a0)         ;    value
			move.w	(a0),d0         ; get number
			and.w	#%01111111,d0   ; (0-127)
			rts

 

Now, calling that, and doing a CMP D0 with a value between 0-127 allows you to get a percentage chance of random.

The VBI code ensures the number is mangled every frame, and using D7 in the random loop means its got a pseudo random seed (assuming d7 is trashed/used as a temp register in the main loop)