1998 Chevy k1500 running poorly
 

I have a 1998 chevy k1500. I bought it with a little under 155xxx miles and ran great. I have only put about 5000 miles on it and all of the sudden it doesnt run right.
Heres my problems, the truck doesnt start right away usually it will turn over multiple times then i have to turn the key off and try again and it will usually start with a couple trys. It will go into gear fine reverse as well. Its still responsive tight away when i give it some gas but if i go past about half throttle the rpms will kick up like its downshifting but the truck will start slowing down kind of like its just going into nuetral. It has a oil leak as of late that looks like it is leaking from around the oil filter. The oil and Transmission fluid are both normal. Im lost on this thing. The truck is sitting in our shop right now and i dont know where to start. I do not wanna start ripping things apart without any idea of what im looking for. Ive been told it could be anything from faulty vacuum lines to a blown transmission. Any information would be great!