01 Grand Am (38k miles) stutters when trying to hold 50 mph. WHY?

just by your description (detailed) id say it could go a few directions ??4 a cause, one way is the 40 mph is near the speed that overdrive is engaged (42 mph) these do sometimes have problems inside them (gears ) also the stutter you mention is sort of an indication that your torque converter didn't lock up or is locking/.unlocking this is an indication of a immanent torque converter failure , if car was made past 2001 then chances are good it has some kind of transmission control system that could log a fault to help you look for a cause like a faulty speed sensor or something , ck your codes !!

Hi Gary:
Checking the spark plugs is a good idea.
How long since you've changed them?

Has the engine light come on yet?

Could be your fluid is low.

My 99 GMC savana 1500 was chugging at like 50 60 mph and gas gauge was stuttering ..it turned out to be computer..I had it re flashed by GMC for 150 bucks ..that fixed all...hope this helps

You need to have the engine control computer scanned for fault codes, if nothing shows then it is very possible you have a failing fuel pump.

Sounds like low fuel pump pressure here ,put a gauge on and see what you get should read 55lbs

try replacing your fuel filter, With big temperature swings this time of year, condensation (water) forms in fuel tank, Prior to changing fuel filter run a bottle of STP gas treatment or a good injector cleaner through it.

sounds like weak fuel pressure or clogged lines, ans/or filter. check these things first, if this is not your problem, id go for a tune up.

check / change the tranny filter to start and check for clutch debrise' in the fuild along with metal shavings