98 Camaro-Please help diagnose

I have a 98 Camaro, 3.8L. Recently I have had a new Borla exhaust put on, new plugs and wires, an oxygen sensor and a mass sensor and some other odds and ends to fix the code that diagnostics picked up...
I have a new problem...
The car starts fine, but as I accelerate the car jerks and the RPM's bounce slightly then smooths out. This only happens in 1st, 2nd and 3rd gear. Once I get the speed above 40 and into 4th or 5th gear it is smooth.
When it idles, it is a bit rough and the air pressure gage which is usually at 60 drops down to 30.
I'd appreciate any advice from anyone.
Thank You very much! :)
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Correction......I put air pressure gage....I mean oil pressure gage.....
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