A million things, help us help you. What does it do? when does it do it? v8 or v6? anything done recently? Overdue service?
Here's a few things to start with :- contaminated fuel (clean out tank, change fuel filter), fouled injectors (check spark plugs & change if required, same with high tension leads) direct fire ignition unit faulty (check coils to start with), collapsed vacuum lines (there are quiet a few to check), air filter obstructed, throttle position sensor & MAP sensor faulty, not enough or too much oil (just change the oil & filter), check & test all Relays, terminals & fuses, also check mains grounding from battery to chassis (use multimeter the check for continuity for all electrics). Disconnect battery before doing any of this! so many other factors could be causing the VN to run rough, when you say this I envision you are on the road driving around & not to be confused with suspension related issues..
