1995 chevy truck 305 engine .shuts off after a few seconds starts back up again put in gear shuts off.

check all fuses. why not? costs nothing .
or RTM
https://eautorepair.net/Marketing/Default.asp

read the manual
next is scan the PCM</TCM at the USA? OBD2 16PIN PORT.
IF THE TCM STORES ERRORS. (I BET IT DID)
MANY MODERN CARS THE TRANSMSSION GOES TO LIMPHOME MODE TO SAVE ITS OWN BACON, AND MORE.

SCAN IT, DO NOT DELAY SCAN IT.
YOU DO OR THE SHOP DOES, TOSS COIN, DO IT.
1: CHECK FUSES
2: CHECK ATF LEVELS PER THE BOOK DO NOT GUESS,DONE HOT ENGINE,IN PARK. PER MANUAL.
3:SCAN IT, NOW. POST ERROR FOR HELP.
4; DO THE PRNDL PRESSURE TESTS IN THE MANUAL (A TOOL)

run fault codes to check for servo/ solenoid problems --TCM faults

the fuel shutoff on the pump is an electric solenoid that shuts off fuel. it could be at fault

Try leaving the gas lid off for about an hour. Also replace fuel filter.

Sounds like it could be a dirty or faulty idle air control valve.Take it off and clean it with a wire brush and a non-chlronated brake cleaner or replace it with a new one.Make sure you reset the computer after cleaning or replacing the valve.

sounds like it is starving for fuel. check or repace fuel filter. when load is on engine, it needs the proper fuel to injectors. check fuel filter.

Have you changed the fuel pump yet?I had the same exact problem.I replaced my fuel pump with an AUTOZONE PIECE OF JUNK fuel pump.That pump lasted almost 2 days.The second day i had it the truck didnt want to start but it did and it ran like ****,and when i put it in gear it ran as long as I didnt touch that gas pedal.You could go get a fuel pressure test or just replace it,your call.You might want to check your filter too but im sure its the pump.Good luck

You need to either replace the ignition lock cylinder or bypass the security system. If you need help with either one, repost.

Check your fuel pump presure. Pump is in the tank. Also try and loosen the fuel cap, the tank vent may be stopped up.

Hook up a fuel pressure guage. Now with the key on and engine off does the fuel pressure come up fast and hold steady at a fixed value?