problem with managment light pops on when driving then goes off..

have checked Tc found o2 sensor fault.Replaced with pattern part,problem with lamp still but fault not as bad as befor..Have cleaned all earth points and checked/cleaned all mulit plugs also given mass airflow new earth point..Was a vauxhal mech for 20 years,not been in trade for 11 years,,so dont have access to updates etc,get help from local small garge so limited knowledge..Fault was when hot about 10 miles lamp pop on ,engine goes flat/no throttle responce..Lift throttle and lamp off and engine ok..seems when under load yet no positve common way to recrate fault only when engine hot/loaded and some times in wet weather..Would appreciate any ideas on matter..Also understand Tech terms Etc..Thank you,,