Anti Theft Alarm

I am the proud owner of a Mercedes 350sd.. a car that I have driven for many many years. However, it sat unused for 6 weeks.. it started first try as always.. but as soon as I put my foot on the brake to engage the gear the anti theft alarm started.. Turned alarm off with the key.. and after several tries I was able to drive away.. but noticed a short beep from the horn whenever I braked. After stopping I tried to start the car again, the same thing happened.. so I disconnected the battery to re-set the computer.. after a couple of re-sets I pulled away without the alarm sounding... a mile down the road the alarm came back on I stopped and took out two fuses #11 & 17 I think, which stopped the alarm and Imanaged to get home. Any ideas anyone? Thank you Paul