I own a 1987 pulsar 1.8sohc. Only sometimes when i drive it the little engine management light comes on. the car seems to loose any punch that it has when the light comes on. does anybody have any ideas to what the problem could be?

Could be a lot of things. Next time the light comes on, after you stop your car look under the passenger seat. You'll see a black box, this is the ECM. Coming off the ECM there is a lead with a connecter block on the end of it, this is the diagnostic link. Put a piece of wire between terminals a and b of the diagnostic link then turn your key to on but don't start the car. Watch the ECM light, it should flash in a sequence that corresponds to a fault code. Read the code by counting the number of flashes
eg for fault code 13 the ECM light would flash once, then pause for on second, then flash another 3 times in quick succession. This sequence would keep repeating until you turn the key off. If you have more than one fault code they will cycle through one after the other.

As for having no power when the light comes on, this is a safety feature to prevent any damage to the engine once the ECM can not read data coming from any particular sensor.

There is a full list of fault codes here:

http://www.alphalink.com.au/~hillsk/delco01.htm