Acura legend question

I recently bought 93 Acura legend LS 4dr around 104k on the odometer.
recently i have some observations on my car for which i hope i will get some suggestions through this forum

a) Some times my car drives like a charm and some times not. I carefully watched and noticed that whenever i fill the gas the driving ability of my car switches to good to bad and vice versa. Normally i fill the tank whenever i go to the gas station. If the car is driving good then it turns bad after filling the tank. If it is driving bad then it goes like rocket and smooth once the tank is filled. First i thought the gas quality is causing this bahavior and started going to the same station everytime but the same behavior continued. It looks crazy for me and i need a help in this matter. Is it the gas quality or weather or the pump static electricity causing some setting on the car ON and OFF? FYI, everytime i buy only premium gas

b) Recently my car started giving clunk sound when auto transmission shift gears from lower to higher and this sound is not coming all the time. I showed it to the local mechanic and he told motor mounts have to be replaced. As a second opinion i took to pepboys and he also told the same but when i took it to the Acura dealer the mechanic told motor mounts are fine and the sounds are common for this old car and showed some other problems like differential seal leakage etc. I am confused and need help from the auto experts in this forum.

Thanks

Does your check engine light ever come on? The gas cap sealing properly affects the way the emission system works. Perhaps a new gas cap will help? $8 to 10 bucks to find out. A cheap troubleshooting item.

I recently bought 93 Acura legend LS 4dr around 104k on the odometer.
recently i have some observations on my car for which i hope i will get some suggestions through this forum

a) Some times my car drives like a charm and some times not. I carefully watched and noticed that whenever i fill the gas the driving ability of my car switches to good to bad and vice versa. Normally i fill the tank whenever i go to the gas station. If the car is driving good then it turns bad after filling the tank. If it is driving bad then it goes like rocket and smooth once the tank is filled. First i thought the gas quality is causing this bahavior and started going to the same station everytime but the same behavior continued. It looks crazy for me and i need a help in this matter. Is it the gas quality or weather or the pump static electricity causing some setting on the car ON and OFF? FYI, everytime i buy only premium gas

b) Recently my car started giving clunk sound when auto transmission shift gears from lower to higher and this sound is not coming all the time. I showed it to the local mechanic and he told motor mounts have to be replaced. As a second opinion i took to pepboys and he also told the same but when i took it to the Acura dealer the mechanic told motor mounts are fine and the sounds are common for this old car and showed some other problems like differential seal leakage etc. I am confused and need help from the auto experts in this forum.

Thanks

Put better gas in it, alott of people put premium then when they sell it the person put in regular, and the engine doesn't run well to the lower change.