ignition problems

my 86 jeep has the gm 2.8 v6 std trans and is carbed.
it idles fine but when you touch the gas the rpms drop then pickup and drop again and continues untill you let off the gas.
when i floor it it revs up normally.
any time the motor is dropping rev's the tach is not working so im guessing it must be somthing electrical.
I was thinking it may be the ignition module but im not sure.
any help would be greatly appreciated

thanks
mike

anyone?