Dont mean to sound cocky or teach you how to suck eggs but a good course of action for this sort of problem is:
1) Check that the servos & power connectors and the crystals are inserted correctly and not corroded (try using small amount of wd40 on the connectors and remove and re-insert them a few times)
2) Replace the batteries in Rx &Tx
3) Eliminate a faulty servo by removing the steering servo connector from the Rx and running the engine (with the wheels off the ground). If the problem is still there then replace the steering servo and do the same with the throttle servo (this can be fun, try getting it to start and run with no throttle control!!!!)
4) If you have two Tx`s try the second one
5) Try changing crystals if you have a spare set
6) Run the car at a different location (in case it picks up interference or somebody else is on same freq).
7) Put electrical tape round the aerial wire from your Rx in case it is shorting out anywhere.
If none of the above work all thats left is you have faulty Rx
I would recommend using a failsafe though. A little money now could result in a lot less spent later
Are you sitting comfortably, then ill tell you a tale
My friend Tx got wet (dont ask) his car (a cen MT2) hit his ankle it then continued on its journey into a kids bike (while the kid was still on it). The bike then fell onto the side of a parked car scratching the paintwork (we refer to this now fondly as the domino accident). All in all it cost him a hefty bill to repair his RC car, repair the back wheel of the kids bike & pay for MY car to be partly resprayed. He also got 16 weeks off work with a broken ankle. The kids parents also threatened to sue him as well.
As for new Tx`s ands Rx`s if you can afford it get a FM system as they are less likely to receive interference. If not try to get a 40MHz AM system rather than a 27MHz as i have found these to be less likely to cause problems.
Its Ok ive finished typing now.
