need help with my celica!!!!-any ideas welcome

i just bought my second celica yesterday

- it's a 1990 toyota celica gts 5spd-

it was sitting for a while. like six monthess, and on the way home -about 50 miles- it was fine but now it jerks and i seem to be waste alot of gas and when i hit the gas it doesnt want to go like normal it seems to lag.

I changed the fuel filter and cleaned the injectors and for a few hours it was fine but its starting again.

any ideas on what it could be?
my dad says maybe the gas tank needs to be replaced-could that be it?