1994 nissan maxima problems?

My car started running poorly with a small drop in rpm, engine vibration and loss of power when getting on gas. This is the second time this has happened. the first time We took vacuum lines off, checked for clogs, and put the lines back. The car ran just fine afterword. This was also after using Lucas injector cleaner for 2 tanks with no result. The car ran fine for about 1500 miles. Most recently we tried the same process with the vacuum lines with no result. My friend suggests I get a Diagnostics which has no guarantee it will even help. Any suggestions on what type of problem this may be or any fixes?