Heating problem

[brian93]

Hi, i recently fitted a new thermostat on my 2002 vtr, since doing this the fan has been cutting in very regular, my heaters are hit and miss sometimes blowing hot air sometimes wont blow hot, the temprature gauge will go over the middle mark but then drop again within seconds then back up n down, engine management light came on yesterday for about 30 mins then went off and aint came on again since, can anyone give me an idea on this? Thanks!