Engine start revs not right what could it be ?

Ive rebuilt the engine and its all bsack together working ok. The only problem is when I start. It will either start as its supposed to do or do one of the following. Slowly increase revs to 2000 then drop down to normal after a short while. Rev upto 1200 revs then slowly die. Either way can be put back to normal if I just blip it a second whilst starting up.
Now ive just received a new collant temp sensor (4.0 thor engine where theres only one on the inlet) and have changed the idle control valve as I thought it was only this. Apart from the temp gauge and the ICV what else could it be. PS did it before engine rebuild so rule out inlet leak.