I need some input please

i just bought a new mustang (91' LX). the problem im having is that when you start the car it wont run on its own for about a minute or two then it does till it starts getting warm and the car starts reving up and down to keep it self running till it eventually stalls. then when you start it back up it will only stay running if you hold over 2000 rpm. if you move your foot off the gas ever so slightley it will stall unless you rev it back over 2000 rpm. in the short time that it does idle by it self if you put it in gear (any gear) it stalls unless you rev it while holding the brake then put it in gear. please help me figure this out, I dont have money to take it to a shop. any suggestions would be great. Thank you Dan

ok before anyone could reply i found part of the problem, i found a vaccum line disconnected and hooked it back up, and also came across a plug wire that was making a bad connection. the next thing i need to tackle is the tach. the tach gives me inaccurate readings sometimes, it says its idleing at 900 rpm then it will jump to 1300 rpm but the engine doesnt rev any faster. so does anyone have any ideas as to how i can make the tach work proper again? and also how does the tach work? is there a plug i can check or something like that?
thanx in advance Dan