issues with oil pressure light..

so i have a 92 jetta gl with 167XXX miles. we did a complete oil change on my car last weekend (new oil and new filter) before we did the oil change my car was fine, well as far as no oil pressure light/buzzer. after the oil change my light/buzzer was going off and staying on the entire time the car was on. i replaced the oil pressure switch and then when i started the car the light would just blink there was no buzzer until i drove the car. i got an oil pressure guage and the pressure seems to be ok at first but after the car is driven for awhile the pressure seems to just get lower and lower to the point where its sitting at 0 when the car is idling... any idea whats going on? im suppose to be driving across state next weekend and only have today to get this fixed... thanks in advanced for the help