missing 3rd gear

[dranger21]

So here's the deal. About 3 months ago I replaced the transmission in my 99 ranger 3.0 and i believe it has the 4R44E transmission in it. It just doesnt want to shift into 3rd gear at higher rpm's. Also when i really stomp on the gas it doesnt seem to want to downshift very quick. My rpms shoot up quick before it drops into gear. Ive had it back in the shop twice since it was replaced to fix the problem because I really have no idea what I'm doing when it comes to transmissions. The shop replaced some valves and it usually runs fine for a few days and then I'm back to where i started. Just today i was told by someone that it could be that the transmission fluid is low but it looks fine on the dipstick or the transmission just needs to be broken in a little. Now I'm no expert but that sounds like a load of bs to me that it just needs broken in a little. If anyone has any idea what the problem could be I'd appreciate any input.
Thanks