86 4runner bucking problem

hey, i have a 1986 4runner and seem to be havign a very annoying
"bucking" problem... it seems to happen after about 5 - 10 mins of
driving but the odd thing is when i shut it off, it runs fine again for
another 5 mins or so... i have had it looked at about 3 times and the
dist. cap, rotars, and plugs were all changed but the problem is still
there... could it be a fuel pressure pump or filter? maybe
electrical..anything would be greatly appreciated!

does anyone think it could be a bad ground at all? I've been trying to
research this problem.