1999 GMC Suburban Will not start

Help my 99 sub will not start, I have tried to spray starting fluid in the intake and will not start. Fuel pump was replaced at 100,000 changed the fuel filter 2 months ago. The fuel pump comes on when the key is turned just like normal. When I try to start the engine it cranks normal for about 4 or 5 seconds then it almost stops cranking then cranks fast again what the heck sounds weird. It does not even try to start. I guess I need to check for spark have not pulled a plug yet. Could this be a clogged cat converter. I have been having problems with the power at about 35 to 40 mph feels like its just chugging along until I drop it into third on the shifting colum, never could figure out what was going on just delt with it. Now this will not start at all. Thanks for your time.