car stuttering in high rpm's

Hey guys, Im going to get my car checked out tomorrow by a friend at a shop, but I just installed an electric autometer oil pressure gauge, and now when I push the car and I go past 3k rpm's its starts sputtering very noticeably, the car drives perfect unless its at wot and it starts at about 3krpm. Oil pressure is right at 25psi at idle and goes up to 50 when cruising, im not sure what it can be. Can it be something electrical? The ground I used for the gauge and the light to the gauge has 4 wires going to the same ground, I dont know what the other two wires are for but I know the car wont run without them grounded cause i've had it loose before and it didnt start, if there are too many wire going to the same ground is it possible it is not getting a good connection and is making the car sputter or am I making up theories here? Any advice? This just started right after I installed the gauge, I checked vacuum lines and everything, I think there is also a SLIGHT surge at idle, maybe like 100rpm barely noticeable.

Oh and another off topic question from my other thread I really need answered, I need to hook up the light wire from the gauge to a light, I was gonna splice it into the ash tray light but that wire is a 18 gauge wire, and the light wire is a 14 gauge, I just need to know that itll be alright to splice the 14 gauge wire into the 18 gauge wire and nothing will blow or catch on fire. Thanks.