2000 durango code p0320/ frustrated

i found the code to mean ign dist engine speed input circut. but what now? what do i have to replace besides the whole vehicle? does anyone out there know? very frustrated with the durango.

In the past 6 months i have replaced the : power steering pump $375.00
dealer oem only

: a/c compressor ebay $100.0
used

: tires $500.00

: transmission fluid filter 100.0

:fan clutch after market
$125.00
:radio $250.00

Still problems with: vibration in rear of vehicle third seat really vibrate

engine codes p0700, p0320 runs like its missing

all dash guages lights go out then come back on

gets about 14 - 16 miles to a gal.