This could be a loose fuse or a bad connector or the fan switch bad wire also but check this out if it's a lever or twist knob switch see if it has looseness in it does the lever move side to side or its not locking in position EX. LOW-MED-HIGH if so then when your hitting bumps the contacts in that switch disconnect and the fan stops working the air bag warning light is on the same circuit probably so there's a loss off either power or the ground and that makes the light come on next there is also a blower motor control module resistor that controls the heater/ac fan under the hood at the fan motor case check that connector and wires try wiggling it and see if you hear the fan stop these are a common problem for 3 reasons (1) mechanics trying to axcess the area can put pressure on this unit trying to get there arms in deep causing the plug to break or a solder joint to break on the circuit board inside it (2) this also can happen from pulling on it to unplug it or reconnecting it to much force on it (3) a wire on it was punctured to test or get power and not sealed causing corrosion to eat part of the wire so intermittent voltage is getting through and as the wire heats up it can't arc and the power break open circuit happens if you can take out the blower motor resistor and if it's abe to be taken apart (release tabs) only don't try to open a completely sealed unit water will get in it and more problems will come anyway if you can open it inspect the circuit board for solder joint breaks where the connector pins are on the board this is a very common problem and a quick resoldering will fix it but also check the whole board a bad soldering joint will show a circle on it in a ripple kind of appearance or just not enough solder on the pins on the connector part or a resistor capacitor inside okay so if you can check it out inside also if the plug connector looks burnt on the face that plugs into the module then there is definitely a problem there with the module or the inside of the plug is broken and wire separation from each other is lost causing wires that shouldn't make contact with each other to arc grounding out but this would most likely blow a fuse but is possible if only a tiny bit of contact happens so what I've explained to you here is definitely your problem you have to investigate and go through the process of eliminating possible causes okay but I would lean more towards the blower motor resistor/plug or a loose fuse in the fuse box obviously I'm not there to check myself and if your truck was worked on recently and this started go to that resistor ok and so you know where this info is coming from I'm a taxi mechanic in nyc.with 50 + years of experience and I've seen a ton of blower motor resistors that have broken solder/plugs causing this problem on/off/on/off from bumpy streets good luck stephen