Hello and welcome to Just answer.com, Super mechanic here. About your vehicle.
That could be the pressrue control solenoid, or some sedimentation in the valve body.
There has been a service bulletin for that type of concern. Here is what it states.
File In Section: 07 - Transmission/Transaxle
Bulletin No.: 0O-07-30-002B
Date: July, 2002
This bulletin is being revised to add additional models and model years. Please discard Corporate Bulletin Number 00-07-30-002A (Section 07 - Transmission/Transaxle).
Some owners of the above vehicles with a HydraMatic 4T65-E transaxle may comment on harsh upshifts or harsh garage shifts, soft shifts, shudders on hard acceleration, or shifts erratic. These conditions may appear intermittently or set a DTC P1811 or P0748. During diagnosis, a low or high line pressure (actual versus desired) may be observed.
The above condition may be due to any one of the following which may affect line pressure output:
Refer to the Line Pressure Check Procedure in SI, along with the Line Pressure Specification Table shown, to determine if actual versus desired pressures are within the values specified. The Scan Tool is only able to control the PC solenoid valve in PARK and NEUTRAL with the vehicle stopped. This protects the clutches from extremely high or low pressures in DRIVE or REVERSE ranges.
Refer to the Line Pressure Specification Table. The pressures in the table assume a temperature of 70°C. The pressure will vary with a change in temperature.
If the actual versus desired pressures are not within the values specified, clean the valve body and replace the PC solenoid valve, if necessary.
Check the PC solenoid valve actual versus desired pressures to verify the new PC solenoid valve is responding correctly.
Refer to the Automatic Transaxle Section of the Service Manual for the proper repair procedure.
Line Pressure Check Procedure Tools Required
Tighten the oil pressure test hole plug to 12 N.m (106 lb in).
Is this a costly repair?
What do you suggest I do next? Should I take it to another mechanic or wait for the dummy light to come on again?