A B717 transitioned from NAV to HEADING while enroute at altitude with no crew input. Later; during arrival vectors the Captain's VIA momentarily failed but recovered while continuing to exhibit small errors.

Narrative

During cruise ATC gave us a vector to the south for spacing. We were then cleared direct the next fix. We did confirm; insert; NAV; and airplane was in a right turn to fix. I looked away momentarily and I saw the airplane in heading mode still in a right turn past the correct heading. I immediately turned the heading bug left back to the fix and told the First Officer the aircraft was not in NAV. I saw no aircraft within 5 miles on TCAS. The Controller said it appears we were turning past the fix and to turn back left to a 200 heading for additional vectors and spacing. He then cleared us direct to the next fix. As a side note; I had also noticed momentarily my #2 DU go off then on; along with my altimeter jumping up several hundred feet in cruise then back to normal close to this time period. Upon arrival at our destination on downwind we experienced what appeared to be a VIA 1 and FMS 1 failure with numerous aural warnings; and my flight and navigation data erratic and unusable; along with several momentary failures and alerts. The First Officers' data seemed to be reliable. I informed ATC we were experiencing some electrical issues; but that I believed we could continue the approach. Somewhere around base to final my data became fairly reliable with some small discrepancies. I regained some NAV display data back after selecting the other flight management computer per checklist. There was a previous write up for VIA failure and I believe this may have been a factor in our heading issue with ATC. Always keep a close eye on the display for any deviations. I have previously seen other aircraft go from NAV to heading for no reason as well as revert from FMS speed.