The Fat Boy and Heatseeker are normally stable rockets and should fly just fine from a standard Estes 1/8" launch rod. If you built your rockets according to the stock plans, they should have the proper cg/cp ratio and be aerodynamically stable if they are leaving the launch rod at the proper velocity.
This unstable behavior typically happens when the rocket leaves the rod with a velocity that is too low for the fins to be effective. This can happen 1.) if the rocket hangs up on the launch rod, 2.) if the launch rod is too short, or 3.) when you are launching on a very windy day, or 4.) the motor is simply too small for the rocket.
In 1.) the rod may be dirty, the launch lug may have some glue or dirrt in it, or the rod could be bent. Any of these conditions can cause the rocket to hang the rod and leave with a lower than normal velocity.
For 2.) most Estes rockets want a 3 ft long launch rod. If you rod is considerably shorter, the velocity leaving the rod may be too low for the fins to be effective.
In 3.), if you use a standard length rod in a strong wind, the fins will stall even if the rocket has its normal velocity due to a high angle of attack caused by the crosswind.
In all of the above cases, the fins have stalled and do not supply a restoring force keep the rocket going straight up.
In 1.) and 2.) the rocket will start to tip over around its cg, but as long as the motor is still firing, the velocity increases and at some point the fins and body provide enough lift. I've seen an Estes V2 take a right hand turn 20 ft above the ground and go into a cruise missile mode for about 200 yards before crashing.
In 3) with a stiff cross-wind, the rocket tips but you get a tumbling aroung the cg as the fins alternately make lift and then stall. This is more probable when the rocket has a 5:1 or less thrust to weight ratio. I have seen a marginally stable rocket spin 4 complete revolutions around its cg at 50 feet above the pad until motor burnout in a 15 mph wind when launched with a standard length rod.
The solutions are simple.
Make sure the rod is clean and straight.
Check to see that the rocket slides freely on the launch rod.
Use a rod at least 3 ft long for the basic Estes rockets. If the wind is above 10 mph, use a 4ft or longer launch rod so the rocket leaves the rod with a higher velocity. Also use motors that provide a 10:1 thrust to weight ratio when the wind is above 10 mph.
Lastly, if you modify your rockets and make them significantly heavier than stock, you may change the cg/cp relationship which reduces stability and you may have to fly it with higher thrust engines to get a stable flight. A 5:1 thrust to weight ratio is the minimum for a standard length rod in winds below 10 mph.