electrical problem?
 

so iv got a few problems that might be conected in a way ill start with the engine light. when i first start my truck it normaly turns on and stays on but when it is off it turns on every time i slow down to about 45 and when i hit the brakes but today my buddy was in the passengers seat and he started rolling the window up and down and the engine light turns off while i was doing about 60 then i let off the petal and the light turns back on then he rolled it down and backup and the engine light turns off? my volts never go below 13v while running. i have 2 semi 950 batterys only about 1 month old. also my tachometer stays between 500 and 1200 rpm and wont go higher.? and my oil pressure gauge will stay on L. i have a 1987 ford f250 xlt lariat 6.9L with around 400,xxx miles on it. :argh: