what car is it?
have u tried the following
get car warm and then disconnect iscv and set base idle to 850rpm then reconnect iscv and as long as fans etc are off idle shd stay constant
get co checked by mot station
again make sure car is warm hook up probe to check co then adjust ur co to the correct level for the chip
clean iscv
and check all connections for any broken wires to iscv/tps dist etc only then after checking simple things take it to someone
engine advantages in essex had a good write up