I need a hand over here...

Ok this is the situation.
A friend of mine has a 1993 suzuki samurai fuel injected.
Arround new year the engine woudn´t start any more , good cranking but no fuel , a local mechanic checked the engine and discovered that the ECM was not sending the (+) to the injector , he run a direct cable from the battery and the engine started.
Got the computer checked and it had problems , changed it for another one (used) and everything was ok until a week ago . As soon as the engine get to operational temperature ( not overheating) y revs up to 2000/2500 and suddenly down to 500. When it cools down everything gets back to normal.

Any ideas?

thanks