Don't expect a lot of performance if you just have a 4 cylinder engine. Perhaps you could run a compression test and computer diagnostics to see if there are any major problems. If you can't find anything wrong, it just might be that you're expecting too much performance from the engine option.
I don't know the average lifespan of a Chevy but i own a 1985 Chevy Capice Classic and it runs perfectly fine.
Time for a tune up.
Check for vacuum leak
If it runs fine on the level but loses power on the hills, check the small fuel filter and if it is OK replace the big fuel filter. Do it yourself for about $10. A dirty filter keeps it from passing enough fuel for a hard pull.
I have a 1990 3.1 and what it turned out to be was the head gasket was leaking now it runs fine
Check your pcv valve or your gas cap :P
check the egr vavle or the purge valve
If your 99 Chevy Suburban runs smooth, but going up hills it loses almost all of its power and only goes 40 to 50 miles per hour, check the distributor. The cap might be broken or cracked.
Will answer my own ? after $340 found problem to be a $13 Temp monitor
some one put the wrong balancer on the crank. or put it on inside out...
if it runs fine until you push the gas pedal the accelerator pump may be bad if it has a carb on it.
If you have an automatic tranny, and the fluid is low, it may not gear down on the hills and may possibly stall. Check this out first.