loss of power going up hills, then SES light comes on

Have a '90 S-10 Blazer 4WD, engine went after 337K miles. Had a "supposedly" used but rebuilt engine put in and had several problems the mechanic couldn't fix, including a problem going up hills. As the truck starts to climb, it all of the sudden loses power, the SES light comes on, and then it struggles the rest of the way...have to floor it to get enough power to make it. Gave up on this engine...just had the mechanic put a newly remanufactured long block in (from ATK), seemed like it was okay, but then had the same "up-hill" problem on the way home from the shop. The mechanic is on vacation for 2 weeks. Any ideas?

weak fuel pump.

Get the codes scanned. They are there for a reason.