94silverado

I have a 94 silverado. Since i have bought this truck i have had a problem with it not starting after it has been runing for a while but it only happens once in a while. When i first bought it the truck only did it about 3 times. after a while 5 monts i had changed everything because the truck had 300,000 miles on it so a complete change out was made back last august new engine new altenator, starter, new battery, cables, trans, radiator (3 core). Also i put in a complete audio video system with new alarm. so now when it is running if am am using everything (TV,dvd,stero,amps,A/C) i notice that the amperage will go down to almost 11 to 12 usually at 14 when only radio. but its also warmer now the last time i checked when it didnt start i let it cool down i could put my hand on the exhaust manifold and it was warm but i could keep my hand there but when i touched the started it was burning hot so i dont know what or where the problem lies. please someone help me!!!!!!!