Mitsubishi Repair Problems? Ask a Mechanic for Answers ASAP
If the belts are physically coming off your only possible scenarios here apart from incorrect installation (not tight enough, too tight, etc) would be a problem with one of the pulleys causing it to toss the belt. While in general we would expect to see an accessory locking up (steering pump pulley seizing intermittently etc), on this particular model it is VERY common for the crank pulley to come loose, to wobble, or for the bolt holding it on to even fail entirely.What I would be doing is checking over all pulley alignments to make sure they are lined up properly particularly the crank pulley.
Second I would put the belt(s) on and check the running condition of the crank pulley (checking for any wobble etc).
Third I would run the vehicle on a lift (in gear, have someone "drive" the truck in the air etc) while I watched the belts for abnormalities until the problem occurred again so I could see exactly what was happening at the time of failure.