Electrical Problem and Motor Comp. Prob.

[nismo240sxer]

Hi, can someone tell me why, while I'm driving, I heard an audible signal 'bing bing bing' like if my door is open or something like that? Is it my door switch that goes short to the ground? And when I shake my steering the sound play again.. so.. is it the key switch who's short or something like that?

2nd: When I'm on the highway and I drove like 110km/h at 2500/3000 rpm, when I remove my foot on the gas pedal the engine is shaking like hell.. until it drops under 2500rpm. So is it a compression problem? The problem only appear between 2500-3000 rpm

My car is a 240sx 1990 with 225k kilo. Thanks