Battery charger - power supply issue affecting ADC

[flotulopex]

Hello,

I'm finishing my 24V lead-acid battery charger and get stuck on a power supply issue.<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=2167&stc=1&d=119651200 3">
The circuit works well but there is a strange (to me) voltage drop on the PIC's side while the current raises on the load and this impacts the ADC measurements.

If no load is connected to the circuit, the controlled output voltage range is from 25V to 30,5Volts and there is a clean constant 5V at the VR1's output (VDD) through the full range.
R2 drops the transformer's voltage from 44VDC to around 40VDC at the LM317K's input (to stay within specs).

When the load is connected, as the charging current raises from 200mA up to 1,2Amps (maximum I need), the VDD drops from 5V to 4,8Volts while the load's voltage is then raising from 25V up to 29Volts.

There is a slowly blinking LED (currently I have one, should be two finally) indicating the loading process goes on (timer toggles the LED every 2 seconds).

I check the differents measured values by the PIC via a serial connection. This is actually how I noticed the voltage drop.

Each time the LED is on, the values differ from the trend.

I naturally thought about an overload on VR1. I can measure between 5,54mA and 14,51mA at the VR1'a output wich is far under it's limit (100mA). I the doubt, I replaced it with à 7805 (1A). Same problem.

For a try, I replaced the actual 22V Zener by a 8,3V one; no change.

I then built up a totally appart 5VDC power supply with a second transformator (230:9VAC). Again, the same voltage drop happens as the load raises (this is more than strange!!!).

Does anyone have an idea what I'm doing wrong here?