Will not run over 40mph, no power,

a 35year old car. never seen one me with less than 10 problems.
100% expected. USA car in OK state clearly told and nice!
NO ENGINE TOLD (size) car is called N13 you generation car.
auto tranny and with 4wd?
USA PULSAR NX, IS 1.6L DOHC OR SOHC ENGINE.
ALL ARE INTERFERENCE ENGINES,
NEVER IGNORE THE TIMING BELT . REPLACE IT FIRST.
so is and engine problem, not transmission failure to up shift, correct>? this fact must be learned first. or off base tests go.
engine Power loss has 50 causes. (electric or pure mechanical)
but in basic engine theory (ASE) and gas engines, and otto cycle engines,
the ordered list is.
1: good engine. . EGR not stuck open. CAM BENT NOT BAD OR WORSE SLIPPING. << FAST DRIVING THIS WILL Bring destruction.
2: good spark ,cap, rotor , HV spark wire set and plugs.
when 40mph cliff of failure hits, is engine misfiring? violently?
or is power loss smooth loss of power, RPM fails to rise more. but falls.
the former is spark bad. (ckp ,cmp or caused by cam belt slip)
#3 fueL
if slow smooth power loss,fuel pressure may be low.
if slow gee? the belt on the cam slips. never drive like that.
day one traded to this car, the belt is first ( I just hate bent valves)

engine lights mostly only report on major problems in engine and transmissions
run the fault codes to check for other sensor problems

Possibly, but it sounds more likely a catalytic converter may need replacing or fuel filter maybe clogged

P0340 code means "Something" is wrong in the timing...
from CMP (Camshaft Position sensor) to the crankshaft bolt.

Very first thing, verify connectors are clean and shiny.
clear all codes, even hard codes.

Could be a tire out of balance, a drive axle, or something in the engine.
Is the check engine light on ?
Would it shake if you were coasting down a hill at 40 or 50 mph ?

run the fault codes
2 possible problems
1 low fuel pressure --pump or tank not venting properly
2--- failed cat converter blocking the exhaust at high rpm

Drive shaft may be loose at differential.Just solved one of these the other day.Could alsoo be worn u-joints. don-ohio

This sounds a lot like a bad TDC (Top Dead Center) sensor. Of the many sensors on this model vehicle, the TDC sensor failure is common and is one of the few sensor failures that will disable your vehicle.

i would start in fuse and relay area with voltmeter.

could be one of a few things,
but firstly the running at 40mph is a safety feature to stop you damaging the engine.
I would guess low oil level, or blocked oil filter.
Does it run at a higher temp. when doing 70 mph for a while, or is it running warmer than normal, that is an indicator of low oil level.
I hope this helps