Engine is bucking in gears 1,2, & 3

I have a 1996 ford ranger 4 cyld 5 spd with 210,000 miles on it. I recently changed the timing belt, fuel filter, and of coarse the oil. My problem is that the engine is bucking in 1st 2nd and 3rd gears when I is under a load (i.e. towing/hauling) or when I get on the gas hard. I don’t think the transmission isn’t slipping because the engine isn’t revving up when the bucking occurs. It is more like the engine is not getting fuel. Maybe the fuel injectors? but the thing is that it doesn’t happen all the time only sometimes and it never happens in 4th or 5th gear(or at least not yet). The problem has been getting progressively worse since it started. I will appreciate anything others have to say or ideas on the problem!!
Thanks!

Listen, if you've changed these things and it is a progressive problem, I would definetely have the fuel pump checked out. Try to go to somebody you can trust.

Yes i could be the fuel pump but you may also whant to check out your catilatic converter it will also cause the engine to buck a high rpm's or under a load.

Hey thanks for the reply! When i bought the truck i bought a new catilatic converter, so i dont think thats it. Ill take it to the shop unless one of you have some idea of how to check the fuel pump? Again, thanks for the reply.

It might be you're spark plugs. Try checking um and make sure the gap is right and that their not filed out.

is it kind of like miss fireing? and is there a check engine light, my truck miss fires and has a check engine light and i checked the code and it told me that it was an open spout circut, i dont know what that means yet, but i do know that when the light is on it uses gas like its a v8 or something, but when the light is off its fine and there isnt any miss fireing. maybe your gettin too much fuel.